Mark E. Hodson

Effects of biochar and the earthworm Eisenia fetida on the bioavailability of polycyclic aromatic hydrocarbons and potentially toxic elements

Polycyclic aromatic hydrocarbons (PAHs) and potentially toxic elements (PTEs) were monitored over 56 days in calcareous contaminated-soil amended with either or both biochar and Eisenia fetida. Biochar reduced total (449 to 306 mg kg(-1)) and bioavailable (cyclodextrin extractable) (276 to 182 mg kg(-1)) PAHs, PAH concentrations in E. fetida (up to 45%) but also earthworm weight. Earthworms increased PAH bioavailability by >40%. Combined treatment results were similar to the biochar-only treatment. Earthworms increased water soluble Co (3.4 to 29.2 mg kg(-1)), Cu (60.0 to 120.1 mg kg(-1)) and Ni (31.7 to 83.0 mg kg(-1)) but not As, Cd, Pb or Zn; biochar reduced water soluble Cu (60 to 37 mg kg(-1)). Combined treatment results were similar to the biochar-only treatment but gave a greater reduction in As and Cd mobility. Biochar has contaminated land remediation potential, but its long-term impact on contaminants and soil biota needs to be assessed.

Fe-sulphate-rich evaporative mineral precipitates from the Río Tinto, southwest Spain

Abstract The soluble metal sulphate salts melanterite, rozenite, rhomboclase, szomolnokite, copiapite, coquimbite, hexahydrite and halotrichite, together with gypsum, have been identified, some for the first time, on the banks of the Río Tinto, SW Spain. Secondary Fe-sulphate minerals can form directly from evaporating, acid, sulphate-rich solutions as a result of pyrite oxidation. Chemical analyses of mixtures of these salt minerals indicate concentrations of Fe (up to 31 wt.%), Mg (up to 4 wt.%), Cu (up to 2 wt.%) and Zn (up to 1 wt.%). These minerals are shown to act as transient storage for metals and can store on average up to 10% (9.5-11%) and 22% (20-23%), Zn and Cu respectively, of the total discharge of the Río Tinto during the summer period. Melanterite and rozenite precipitates at Río Tinto are only found in association with very acidic drainage waters (pH <1.0) draining directly from pyritic waste piles. Copiapite precipitates abundantly on the banks of the Río Tinto by (1) direct evaporation of the river water; or (2) as part of a paragenetic sequence with the inclusion of minor halotrichite, indicating natural dehydration and decomposition. The natural occurrences are comparable with the process of paragenesis from the evaporation of Río Tinto river water under laboratory experiments resulting in the formation of aluminocopiapite, halotrichite, coquimbite, voltaite and gypsum.

Do earthworms impact metal mobility and availability in soil? - A review

The importance of earthworms to ecosystem functioning has led to many studies on the impacts of metals on earthworms. Far less attention has been paid to the impact that earthworms have on soil metals both in terms of metal mobility and availability. In this review we consider which earthworms have been used in such studies, which soil components have been investigated, which types of soil have been used and what measures of mobility and availability applied. We proceed to review proposed reasons for effects: changes in microbial populations, pH, dissolved organic carbon and metal speciation. The balance of evidence suggests that earthworms increase metal mobility and availability but more studies are required to determine the precise mechanism for this.

Changes in the leachability of metals from dredged canal sediments during drying and oxidation.

The behaviour of metals in canal sediments after their disposal to land has important implications for the environmental management of canal dredgings. The leaching behaviour of trace metals was investigated in a laboratory-based experiment using sediment from a canal in the UK (139 mg Zn kg-1dry sediment, 1.1 mg Cd, kg-1dry sediment 31.5 mg Cr kg-1dry sediment, 20.6 mg Cu kg-1dry sediment 48.4 mg Ni kg-1dry sediment, 43.4 mg Pb kg-1dry sediment and 7.6 mg As kg-1dry sediment). The sediment was allowed to dry. Cores (10 cm long) of the drying canal sediment were taken over a period of 12 weeks. A simple water extraction procedure was used to investigate changes in metal leachability at varying depths through the cores. Metal leachability increased over the first five weeks of drying and then subsequently decreased between weeks five and twelve, (e.g. Cd increased from approximately 0.006 to 0.018 mg/kgsediment then decreased to approximately 0.006 mg/kgsediment, Zn increased from approximately 1.5 to 3 mg/kgsediment and then decreased to approximately 1.5 mg/kgsediment). These results were combined with sulphide/sulphate ratios, which showed a decrease as the sediment dried (e.g. at 2-4 cm depth from approximately 1 to 0.49), and BCR sequential extraction data. Most metals (except Cd and As) showed a redistribution from the residual phase into more mobile phases as the sediment dried and oxidised. Metal leachability was strongly correlated with the sulphide/sulphate ratio with leachability normally increasing with decreasing sulphide/sulphate ratio. The combined results were used to infer the likely behaviour of dredged material upon disposal to land.

Experimental evidence for mobility of Zr and other trace elements in soils

A Soxhlet extraction was carried out over a period of 27 d on a column comprising 3 cm of quartz overlain by 4 cm of soil from the B horizon and then 1 cm of soil from the A horizon of a granitic podzol. Major and trace elements were leached from the column and accumulated in a reservoir at the base of the column. Total loss of elements from the soil over the course of the experiment ranged from 0.002 to 1 wt% with major elements and the light and heavy rare earth elements (REE) showing the largest percentage losses. Zirconium (0.002%) and then Al (0.008%) showed the lowest percentage loss. The light REE were leached out of the soil preferentially to the mid REE. All elements showed accumulation, by a factor of 2 to 11, in the quartz layers at the base of the column, particularly in the upper first 1 cm of the quartz. Major elements were leached from the column at a rate of 0.02 to 0.59 μmol h−1 whereas Zr, Nd, Sm, Gd, Dy, Rb, and Sr were leached at the rate of 0.5 to 30 × 10−6 μmol h−1. Concentrations of other REE in the reservoir increased over the duration of the experiment, but they were poorly correlated with time, so leaching rates were not calculated. Normalization of the major element leaching rates to take into account the constant flushing of water through the column, the average annual rainfall in the Allt a’Mharcaidh catchment in Scotland from where the soil was sampled, and the cross-sectional area of the soil in the column, together with the temperature of the soil in the column (70°C) compared with the average annual temperature of the Allt a’Mharcaidh catchment (5.7°C), gave major element release rates from the soil of 0.002 to 0.97 mEq m−2 yr−1 (depending on the choice of Ea, the dissolution activation energy), which are generally less than those measured in the field of 0.1 to 40.9 mEq m−2 yr−1. Calculations showed that despite the redistribution and loss of Zr from the column, assumptions of Zr mobility would have had a negligible effect on calculated element release rates of Na, Ca, Fe, and Mg. However, significant underestimates of the release of K (5%), Ti (57%), Al (5%), and Si (10%) as well as some trace elements (e.g., Nd, 23%; Rb, 54%; Sr, 24%) would have occurred. Concentrations of Ca and Sr leached from the column correlated well (RSQ = 0.93, p < 0.01), supporting the idea of the use of Sr release as a proxy for Ca release in weathering rate calculations. The release rates and percentage loss of REE from the soil varied between elements indicating that REE distribution patterns of rocks and soils may not be preserved in drainage waters.

Effect of bone meal (calcium phosphate) amendments on metal release from contaminated soils - a leaching column study

Metal-contaminated soil may be remediated in situ by the formation of highly insoluble metal phosphates if an appropriate phosphorus (P) source can be found. Leaching column experiments have been carried out to assess the suitability of bone meal as such a source. Bone meal additions reduced metal release from a contaminated soil, increased soil and leachate pH and decreased soil leachate toxicity. Minimal P leaching occurred from the soil. The data are consistent with a proton consuming bone meal (calcium phosphate) dissolution reaction followed by the formation of metal phosphates. Although, no metal phosphates were observed to form using X-ray diffraction of scanning electron microscopy this could be due to their low concentration. Relatively low (1:50 bone meal:soil) concentrations of fine (90-500 microns) bone meal would appear to be an effective treatment for metal-contaminated soils.

The role of intragranular microtextures and microstructures in chemical and mechanical weathering: Direct comparisons of experimentally and naturally weathered alkali feldspars

Abstract Electron microscopic observations of alkali feldspars from soils show that intragranular microtextures, such as exsolution lamellae, and microstructures, primarily dislocations, are both highly significant determinants of the weathering behaviour of these minerals. In particular, strained structure around intersecting edge dislocations in the plane of exsolution lamellae, ∼( 6 01), dissolves at a rate which is orders of magnitude greater than unstrained feldspar, producing a mesh of intersecting etch tubes extending >5 × 10−3 cm into the crystal. As a result, dissolution at dislocations is the major source of solutes during initial stages of chemical weathering in the field. With progressive chemical weathering, the most highly reactive feldspar is consumed by growth and coalescence of etch tubes, but outer parts of the grain are physically weakened, leading to mechanical flaking that increases available surface area and exposes further reactive sites. In contrast, previous dissolution experiments, and microscopy of reacted surfaces, have shown little or no correlation between dissolution rate and dislocation density and few visible signs of dissolution at particularly reactive sites. To resolve the apparent discrepancy between field and laboratory behaviour we have carried out flow-through dissolution experiments using pH 2 HCl at 25°C on three alkali feldspars with carefully characterized intragranular microtextures and microstructures. These alkali feldspars were: (1) Eifel sanidine, an alkali feldspar that has no microtextures at the TEM scale and a low dislocation density ( 2–3 × 108 cm−2), and (3) naturally weathered alkali feldspars, also from the Shap Granite, which have the same microtextures as unweathered Shap Granite alkali feldspars but, because they have been weathered, have a lower density of dissolution reactive dislocations exposed on grain surfaces (

Measuring and modelling mixture toxicity of imidacloprid and thiacloprid on Caenorhabditis elegans and Eisenia fetida

While the standard models of concentration addition and independent action predict overall toxicity of multicomponent mixtures reasonably, interactions may limit the predictive capability when a few compounds dominate a mixture. This study was conducted to test if statistically significant systematic deviations from concentration addition (i.e. synergism/antagonism, dose ratio- or dose level-dependency) occur when two taxonomically unrelated species, the earthworm Eisenia fetida and the nematode Caenorhabditis elegans were exposed to a full range of mixtures of the similar acting neonicotinoid pesticides imidacloprid and thiacloprid. The effect of the mixtures on C. elegans was described significantly better (p<0.01) by a dose level-dependent deviation from the concentration addition model than by the reference model alone, while the reference model description of the effects on E. fetida could not be significantly improved. These results highlight that deviations from concentration addition are possible even with similar acting compounds, but that the nature of such deviations are species dependent. For improving ecological risk assessment of simple mixtures, this implies that the concentration addition model may need to be used in a probabilistic context, rather than in its traditional deterministic manner.

Micropore surface area variation with grain size in unweathered alkali feldspars: implications for surface roughness and dissolution studies

Abstract The BET and micropore surface areas of different size fractions of four different unweathered alkali feldspars were calculated using gas adsorption data. BET surface area decreased with increasing grain size. Micropore surface area accounted for 12–84 % of BET surface area. For two of the feldspars, Eifel sanidine and KB14, micropore surface area increased with decreasing grain size; with regard to the other two, Perth perthite and Keystone microcline, surface area varied randomly with grain size. Micropores have diameters ≤ 2 nm, which is on the same scale as alkali feldspar unit-cell dimensions. As there are no channels in the alkali feldspar crystal structure, it was concluded that the measured micropore surface area was due to cracks generated during the grinding involved in sample preparation. Values for the surface roughness of the grains were calculated assuming that micropore surface area equaled internal surface area. Values varied between 1 and 7 for the different grain size fractions of Perth perthite, Keystone microcline, and Eifel sanidine. For KB14 surface roughness varied between 7 and 24. It is possible that the dissolution of feldspars proceeds at different rates down cracks and at the outer surface of grains. If this is the case then the present data suggest that care must be taken when comparing surface area normalised dissolution rates obtained from chemically similar samples which have been ground for different periods of time with different degrees of force.

Passive Samplers Provide a Better Prediction of PAH Bioaccumulation in Earthworms and Plant Roots than Exhaustive, Mild Solvent, and Cyclodextrin Extractions.

A number of extraction methods have been developed to assess polycyclic aromatic hydrocarbon (PAH) bioavailability in soils. As these methods are rarely tested in a comparative manner, against different test organisms, and using field-contaminated soils, it is unclear which method gives the most accurate measure of the actual soil ecosystem exposure. In this study, PAH bioavailability was assessed in ten field-contaminated soils by using exhaustive acetone/hexane extractions, mild solvent (butanol) extractions, cyclodextrin extractions, and two passive sampling methods; solid phase micro extraction (SPME) and polyoxymethylene solid phase extraction (POM-SPE). Results were compared to actual PAH bioaccumulation in earthworms (Eisenia fetida) and rye grass (Lolium multiflorum) roots. Exhaustive, mild solvent and cyclodextrin extractions consistently overpredicted biotic concentrations by a factor of 10-10 000 and therefore seem inappropriate for predicting PAH bioaccumulation in field contaminated soils. In contrast, passive samplers generally predicted PAH concentrations in earthworms within a factor of 10, although correlations between predicted and measured concentrations were considerably scattered. The same applied to the plant data, where passive samplers also tended to underpredict root concentrations. These results indicate the potential of passive samplers to predict PAH bioaccumulation, yet call for comparative studies between passive samplers and further research on plant bioavailability.