Emma F. Covelo

Effects of tree vegetation and waste amendments on the fractionation of Cr, Cu, Ni, Pb and Zn in polluted mine soils

Soils at a depleted copper mine in Touro (Galicia, Spain) are physically and chemically degraded and have also polluted the surrounding area. Due to these environmental problems and the large area of these mine soils, the reclamation strategies carried out at Touro have consisted of planting trees (pine or eucalyptus), amending with waste material (sewage sludge and paper mill residues), or using both treatments. Tree planting has been carried out for 21 years and waste amending for 10. Two different zones were selected in the mine (the settling pond and mine tailing) in order to evaluate the effect of the different reclamation practices on the chemical fractions of Cr, Cu, Ni, Pb and Zn. The results showed that soils in the untreated sites were polluted by Cr and Cu. Planting pines and eucalyptus on mine soils decreased the concentration of these heavy metals in non-mobile soil fractions. Amendments also attenuated pollution by Cr and Cu as the wastes that were used had lower concentrations than the untreated mine soils. Planting trees increased Ni, Pb and Zn retention in the non-mobile fractions, preventing them from being leached into surrounding areas. However, caution should be exercised when adding organic wastes, as they can lead to increase concentrations of Ni, Pb and Zn and their phytoavailable form. The results also showed that changes in the chemical fractionation of heavy metals in soils was more influenced by the clay percentage and both dissolved and soil organic carbon (SOC and DOC) than by soil pH or cation exchange capacity.

Sorption and desorption of Cd, Cr, Cu, Ni, Pb and Zn by a Fibric Histosol and its organo-mineral fraction.

It has often been stated that the contribution of soil organic matter (OM) to the sorption of heavy metals can be evaluated using the surface horizon of a Histosol as typical of soil organic matter. However, components of Histosols other than organic matter, such as clay minerals and Fe or Mn oxides, can also sorb heavy metals. In this work we compared the heavy metal sorption and desorption behaviour of a Fibric Histosol H horizon with that of its organo-mineral fraction (OMF, defined as the fraction of wet particle size <100 microm) in experiments in which Cd, Cr, Cu, Ni, Pb and Zn were sorbed simultaneously from solutions of various concentrations. The OMF sorbed the metals reversibly and apparently mainly at specific sites to each particular metal, in keeping with the good fit of Langmuir isotherms to the sorption data; greatest sorption capacity was for lead and copper. Whole H horizon appeared to include sites at which binding was less reversible and chromium competed with the other metals, especially copper. Organo-mineral fraction is suggested to evaluate the soil organic matter contribution to heavy metal fixation.

A versatile parameter for comparing the capacities of soils for sorption and retention of heavy metals dumped individually or together : Results for cadmium, copper and lead in twenty soil horizons

Heavy metals can be immobilized by soils and their distribution among the particulate soil components depends on the identity and amount of the metal, the properties of the soil, and other environmental factors. Cd, Cu and Pb are among the most potentially toxic heavy metals, are present--often together--in numerous polluting spills and in agrochemicals. We evaluated the individual and competitive sorption and retention of Cd, Cu and Pb on 20 soil horizons. As is usual, the isotherms constructed were so irregular, especially the retention isotherms, that it was not possible to classify and compare them in terms of the conventional isotherm shapes. Nor could they be compared using Langmuir or Freundlich parameters, since not all could be fitted with either of these equations. They were therefore characterized and compared in terms of several varieties of distribution coefficient, including a novel adimensional parameter K(r) which on the basis of correlation and principal components analyses was judged to be the most coherent and generally applicable to all experimental conditions (sorption and desorption starting from single- or multi-metal solutions). K(r) proved to be mainly determined by soil pH, effective cation exchange capacity, and Mn oxides content.

Tree vegetation and waste amendments to improve the physical condition of copper mine soils

Mine soils are often physically degraded, which hinders plants development. The untreated soils at the depleted copper mine in Touro (Galicia, north-west Spain) have no vegetation and are probably physically degraded. These mine soils were reclaimed both by planting trees and amending with waste (sewage sludge and paper mill residues). The purpose was to determine the effect of these treatments on the physical quality of the soils of the Touro mine under field conditions. We evaluated the physical situation of both the settling pond and the mine tailings in Touro, then comparing them with their respective treated areas: vegetated, amended or with both treatments at the same time. We corroborated that the physical degradation of untreated soils was considerable: low porosity, high density and less than 50% of water stable aggregates. The trees that were planted increased porosity, probably due to root activity. The added amendments increased the mean weight diameter (MWD), the percentage of water stable aggregates (WSA) and the stability index (SI) due to the high organic carbon content in the added wastes. We verified that using both treatments at the same time is better than using only one to improve the physical situation of mine soils.

Effects of sewage sludge and barley straw treatment on the sorption and retention of Cu, Cd and Pb by coppermine Anthropic Regosols.

To evaluate the involvement of cation exchange in the competitive and separate sorption and retention of Cu(2+), Cd(2+) and Pb(2+) by soils developing from a copper mine spoil, and to determine the effects of sludge and barley straw treatment on the intensity and reversibility of sorption and retention, isotherms were constructed by means of batch sorption/desorption experiments in which displaced Ca(2+), Mg(2+), K(+) and Al(3+) were also determined. Amendment with sludge and barley straw was associated with an increase in pH of about 4 units; approximately 75-, 1900- and 55-fold increases in CEC(e), organic matter content and Mn oxides content, respectively; and greatly increased capacity for the sorption and retention of Pb, Cu and Cd. Most heavy metal sorption came about through displacement of the predominant cation in the exchange complex (Al(3+) in unamended soils, and Ca(2+) in amended soils), but the greater total sorption from multi-metal solutions also involved the displacement of other exchangeable cations. The parameter K(r) clearly reflected the lower sorption and retention capacities of unamended minesoils (K(r)<0.2 for all three metals, as against K(r) approximately 0.54 (Cd) or K(r)>0.97 (Pb and Cu) for amended minesoils); the competition for sorption sites in multi-metal solutions (for any given metal, the K(r) for single-metal solutions was invariably greater than the corresponding K(r) for multi-metal solutions); and the order of preference among metals for sorption and retention (Pb>Cd>or=Cu for sorption on unamended soils, which had virtually no organic matter, an important Cu-binding component; Pb>Cu>or=Cd otherwise). The values of the hysteresis index HI were likewise in agreement with previous results on the reversibility of the sorption of these metals, identifying Pb and Cd as the most and the least irreversibly sorbed metals, respectively. The amendment combination investigated successfully increased the immobilization of Pb, Cu and Cd by this minesoil, but a change in the amendment dosage is necessary in order to achieve near-neutral pH and minimize the predominance of Ca(2+) in the exchangeable cation complex.

Soil management of copper mine tailing soils — Sludge amendment and tree vegetation could improve biological soil quality

Mine soils at the depleted copper mine in Touro (Northwest Spain) are physico-chemically degraded and polluted by chromium and copper. To increase the quality of these soils, some areas at this mine have been vegetated with eucalyptus or pines, amended with sludges, or received both treatments. Four sites were selected at the Touro mine tailing in order to evaluate the effect of these different reclamation treatments on the biological soil quality: (1) Control (untreated), (2) Forest (vegetated), (3) Sludge (amended with sludges) and (4) Forest+Sludge (vegetated and amended). The new approach of the present work is that we evaluated the effect of planting trees or/and amending with sludges on the biological soil quality of mine sites polluted by metals under field conditions. The addition of sludges to mine sites recovered the biological quality of the soil, while vegetating with trees did not increase microbial biomass and function to the level of unpolluted sites. Moreover, amending with sludges increased the efficiency of the soils microbial community to metabolize C and N, which was indicated by the decrease of the specific enzyme activities and the increase in the ratio Cmic:Nmic (shift towards predominance of fungi instead of bacteria). However, the high Cu and Cr concentrations still have negative influence on the microorganisms in all the treated soils. For the future remediation of mine soils, we recommend periodically adding sludge and planting native legume species.

Competitive Adsorption and Desorption of Cadmium, Chromium, Copper, Nickel, Lead, and Zinc by Humic Umbrisols

Abstract The objective of this study was the evaluation of the selectivity sequences of adsorption and retention of cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) in four Humic Umbrisols from the province of Pontevedra, Spain. The characteristics of the surface horizons of the soils showed significant differences in the components and properties that influence adsorption capacity and mobility of metals and, therefore, their retention by the soils. The most adsorbent and retentive soil after desorption has the highest content of manganese (Mn), iron (Fe), and aluminum (Al) oxides. The least adsorbent has the lowest content of oxides and organic matter. The metal adsorbed in greater amount by the soils was Pb, followed by Cu and Cr, which competed favorably for adsorption sites with Cd, Ni, and Zn. After desorption, Cr is the most retained metal. The greater retention of Cr seems to indicate that the relation charge-to-radius is the most influential factor in the retention of this element. In general, the properties and components of the soils have more influence on the adsorption and retention of the metals than the properties of the metallic ions.

Enrichment of marsh soils with heavy metals by effect of anthropic pollution

The impact of waste disposal on marsh soils was assessed in topsoil samples collected at eight randomly selected points in the salt marsh in Ramallosa (Pontevedra, Spain) at 4-month intervals for 2 years. Polluted soil samples were characterized in physico-chemical terms and their heavy metal contents determined by comparison with control, unpolluted samples. The results revealed a marked effect of waste discharges on the soils in the area, which have low contents in heavy metals under normal environmental conditions. In fact, the studied soils were found to contain substantial amounts of total and DTPA-extractable Cd, Cu, Pb and Zn. Based on the relationship of the redox potential with the DTPA-extractable Cd, Cu, Pb, and Zn contents of the soils, strongly reductive conditions raised the total contents in these elements by effect of their remaining in the soils as precipitated sulphides. Such contents, however, decreased as oxidative conditions gradually prevailed. The contents in DTPA-extractable metals increased with increasing Eh through the release of the metals in ionic form to the soil solution under oxidative conditions. The contents in heavy metals concentrating in the polluted soils were several times higher than those in the control soils (viz. 2 vs. 6 for Cd, 4 vs. 6 for Cu, 4 vs. 20 for Pb, and 2 vs. 15 for Zn, all in mgkg(-1)). This can be expected to influence the amounts of available heavy metals present in the soils, and hence the environmental quality of the area, in the near future. Based on its geoaccumulation index (Class >/=3 for Cd and Cu, and 1-4 for Pb and Zn), the Ramallosa marsh is highly polluted with Cd and moderately to highly polluted with Cu, Pb and Zn. The enrichment factors obtained confirm that the salt marsh is highly polluted (especially with Cd) as the primary result of anthropic activity.

Classification and regression trees (CARTs) for modelling the sorption and retention of heavy metals by soil

The sorption and retention of mixtures of heavy metals by soil is a complex process that depends on both soil properties and competition between metals for sorption sites. In this study, the sorption and retention of mixtures of Cd, Cr, Pb, Cu, Zn and Ni by a representative sample of soils from Galicia (N.W. Spain) was reproduced considerably more precisely by binary decision-tree regression models constructed using the CART algorithm than by linear regression models. Of the six metals competing for sorption sites in these experiments, Pb, Cu and Cr were sorbed and retained to a greater extent than Cd, Ni and Zn. Non-linear tree regression models constructed with CART fitted the data better than linear models, especially for Cd, Ni and Zn; and with both kinds of model the data for Pb, Cu and Cr were fitted better than those for Cd, Ni and Zn (the difference being much more marked for linear models), suggesting that the influence of soil properties on the sorption and retention of the latter three metals was limited by the preferential binding of the former three.

Phytoremediation of amended copper mine tailings with Brassica juncea

Mine tailings remain a major concern to the mining industry and the environment. This study evaluates the potential of Brassica juncea for phytoremediation of copper mine tailings treated with technosol (TE) and compost (CO) and the effect of these amendments in the process. Ecophysiological and biochemical results reveal the capacity of B. juncea to grow and develop under the influence of both amendments, and aspects like its soil-covering ability, vigorous seedling growth, tolerance to metal toxicity and potential immobilisation of excluded trace metals in the rhizosphere, denote the species aptitude for phytostabilisation. Moreover, B. juncea also exhibits fitness for the uptake of Cu and Zn, due to its high biomass production and striking translocation and bioconcentration ratios for these metals, thus qualifying this species as a good candidate for phytoextraction in similar mine tailings. The treatments depict different effects on the phytoremediation process, with TE favouring phytostabilisation and CO enhancing phytoextraction.