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

Mineralogical and sorption characterization of lateritic soils from Southwestern Nigeria for use as landfill liners

Lateritic soils are prevalent in the tropical regions, and they are used for various construction purposes including landfill liner applications. However, their contaminant attenuation potentials through sorption and the influence of parent rocks on this property are poorly understood. This study investigates lateritic soils from southwestern Nigeria as barrier to leachate migration in engineered landfills and related waste containment facilities. The lateritic soils were investigated through X-ray diffraction (XRD), geochemical analysis and batch equilibrium sorption test to evaluate the competitive sorption of Mn, Cd, Pb, Cu and Cr which are common in landfill leachates. The XRD analysis shows that the kaolinite and dickite are the dominant clay minerals present in the lateritic soils, implying low desiccation cracking and low shrink-swell behaviour. The geochemical analysis indicate that the lateritic soils are silico-alumino-feruginuous with average major oxide composition of SiO2, Al2O3and Fe2O3 of 50.86 wt%, 29.83 wt% and 14.29 wt%), respectively. Additional oxides with lower abundance include TiO2 (1.55 wt%), Na2O (0.01 wt%), MgO (0.36 wt%), CaO (0.15 wt%) and K2O (1.52 wt%). The lateritic soils contain trace amount of heavy metals with average concentrations of Cd (0.039 ppm), Pb (0.548 ppm), Cr (0.189 ppm), Cu (0.964 ppm), Mn (0.145 ppm). Furthermore, the low abundance of sodium oxide in the lateritic soils indicates that the soil particles are not susceptible to dispersion while the presence of considerable amount of iron and manganese oxides signify its good heavy metal retention. The batch equilibrium sorption analysis shows that the lateritic soils derived from granite-gneiss and charnockite exhibit better sorption potential than those derived from schist and quartzite. This high sorption capacity is intricately related to the presence of goethite in the soils. The sorption of these trace metals onto the lateritic soils follows Langmuir type isotherm and these isotherms deviate from the corresponding desorption isotherms to different degrees indicating various extents of hysteresis. The sorption hysteresis indices for these trace metals range from 0.63 to 0.99 and imply that the trace metals may re-leached to the surrounding soils and groundwater. Thus, it is recommended that landfill liners utilizing these lateritic soils are design as a composite containment facility by integrating compacted soil liners, leachate collection systems and monitoring networks to ensure effective environmental protection.

Use of Three Different Nanoparticles to Reduce Cd Availability in Soils: Effects on Germination and Early Growth of Sinapis alba L

Globally, cadmium (Cd) is one of the metals that causes the most significant problems of contamination in agricultural soils and toxicity in living organisms. In this study, the ability of three different nanoparticles (dose 3% w/w) (hydroxyapatite (HANPs), maghemite (MNPs), or zero-valent iron (FeNPs)) to decrease the availability of Cd in artificially contaminated agricultural soil was investigated. The effect of Cd and nanoparticles on germination and early growth of Sinapis alba L. was also assessed by tolerance/toxicity bioassays. The available Cd contents in the contaminated soil decreased after treatment with the nanoparticles (available Cd decreased with HANPs: >96.9%, MNPs: >91.9%, FeNPs: >94%), indicating that these nanoparticles are highly efficient for the fixation of available Cd. The toxicity/tolerance bioassays showed different behavior for each nanoparticle. The HANPs negatively affected germination (G(%): 20% worsening compared to control soil), early root growth (G_index: -27.7% compared to control soil), and aerial parts (Ap_index: -12%) of S. alba, but showed positive effects compared to Cd-contaminated soils (G_index: +8-11%; Ap_index: +26-47%). MNP treatment in Cd-contaminated soils had a positive effect on germination (G(%): 6-10% improvement) and early growth of roots (G_index: +16%) and aerial parts (Ap_index: +16-19%). The FeNPs had a positive influence on germination (G(%): +10%) and growth of aerial parts (Ap_index: +12-16%) but not on early growth of roots (G_index: 0%). These nanoparticles can be used to reduce highly available Cd contents in contaminated soils, but MNPs and FeNPs showed the most favorable effects on the early growth and germination of S. alba.

An efficient biochar synthesized by iron-zinc modified corn straw for simultaneously immobilization Cd in acidic and alkaline soils

Synthetic functional biochar using agricultural waste as raw materials not only serves as an effective means for recycling waste but can also be employed for the remediation of heavy metal contaminated soil. However, the associated effect and mechanism underlying the immobilization of functional biochar in acidic and alkaline soils remain unclear. In this study, a novel iron-zinc oxide composite modified corn straw (Fe/Zn-YBC) was prepared and applied for the remediation of cadmium-contaminated acidic and alkaline farmland soils. The results showed that the addition of Fe/Zn-YBC increased the pH, cation exchange capacity (CEC), and dissolved organic carbon (DOC) in acidic soil, while increased the pH and DOC in alkaline soil. After immobilization for 42 d, the DTPA-Cd content in acidic and alkaline soils treated with Fe/Zn-YBC decreased by 12.77 %-57.45 % and 23.73 %-52.50 %, respectively. Fe/Zn-YBC treatment promoted the transformation of the exchangeable fraction into the Fe/Mn oxyhydroxide fraction of Cd, and increased the abundance and diversity of bacterial communities in the two soils. Furthermore, the SEM-EDS, XRD and FTIR results for Fe/Zn-YBC separated from the test soils showed that the distribution of Cd adsorbed on Fe/Zn-YBC was positively correlated with Fe, Zn, and O. Additionally, the Cd complexes (CdCO₃, CdZnFe₂O₄ and CdO) detected on Fe/Zn-YBC indicated that the primary immobilization mechanism of Fe/Zn-YBC involved the complexation of Cd and Fe, Zn oxides, and the precipitation of Cd and CO₃^2- in acidic and alkaline soils. The efficient remediation capacity and associated mechanism for this novel functional biochar provide insights for improved remediation of heavy metal contaminated farmland soil.

The risk assessment of inorganic and organic pollutant levels in an urban area affected by intensive industry

The city of Litvínov (North Bohemia, Czech Republic) is seriously affected by coal mining, coal processing, and intensive industrial activities. Within the urban area, the potential environmental hazard of risk elements (in soil and vegetation) and polycyclic aromatic hydrocarbons (PAHs, in soils) was estimated using selected environmental and human health hazard indices. In total, 24 sites were sampled, including the city center, residential areas, industrialized zone, and areas close to operating and/or abandoned coal mines. The results showed elevated values of As, Cd, Cu, Ni, Pb, and Zn in soils (the maximum levels of individual pollution indices varied between 3 and 5 for As, Pb, and Zn); the risk assessment code (RAC) values indicated high bioaccessibility of Cd and Zn. The high mobility of Cd was confirmed by their bioaccumulation factors (BAF) in the aboveground biomass of Taraxacum sect. Ruderalia and Polygonum aviculare, reaching up to 1.9 and 2.9, respectively. The Cd content in plants presents a substantial health hazard for herbivores such as Oryctolagus cuniculus living within the urban area. The PAH levels in the soils also showed elevated values; the contents of benzo(a)pyrene exceeded more than 2-fold the indicative values for potential health risk for biota, especially near the abandoned coal mines. The incremental lifetime cancer risks (ILCR) for ingestion of the contaminated soil showed only low or negligible cancerogenic risk, varying between 6.7 × 10^-7 and 1.6 × 10^-5 for children, and between 9.9 × 10^-7 and 2.7 × 10^-6 for adults. However, the potential health impact of the inhalation of the contaminated particulate matter should be included in the further research. Although the contamination level in the investigated area does not represent an imminent environmental and health risk, the potential remediation measures should be considered in the future.

Efficacy of Enzymatically Induced Calcium Carbonate Precipitation in the Retention of Heavy Metal Ions

This study evaluated the efficacy of enzyme induced calcite precipitation (EICP) in restricting the mobility of heavy metals in soils. EICP is an environmentally friendly method that has wide ranging applications in the sustainable development of civil infrastructure. The study examined the desorption of three heavy metals from treated and untreated soils using ethylene diamine tetra-acetic acid (EDTA) and citric acid (C6H8O7) extractants under harsh conditions. Two natural soils spiked with cadmium (Cd), nickel (Ni), and lead (Pb) were studied in this research. The soils were treated with three types of enzyme solutions (ESs) to achieve EICP. A combination of urea of one molarity (M), 0.67 M calcium chloride, and urease enzyme (3 g/L) was mixed in deionized (DI) water to prepare enzyme solution 1 (ES1); non-fat milk powder (4 g/L) was added to ES1 to prepare enzyme solution 2 (ES2); and 0.37 M urea, 0.25 M calcium chloride, 0.85 g/L urease enzyme, and 4 g/L non-fat milk powder were mixed in DI water to prepare enzyme solution 3 (ES3). Ni, Cd, and Pb were added with load ratios of 50 and 100 mg/kg to both untreated and treated soils to study the effect of EICP on desorption rates of the heavy metals from soil. Desorption studies were performed after a curing period of 40 days. The curing period started after the soil samples were spiked with heavy metals. Soils treated with ESs were spiked with heavy metals after a curing period of 21 days and then further cured for 40 days. The amount of CaCO3 precipitated in the soil by the ESs was quantified using a gravimetric acid digestion test, which related the desorption of heavy metals to the amount of precipitated CaCO3. The order of desorption was as follows: Cd > Ni > Pb. It was observed that the average maximum removal efficiency of the untreated soil samples (irrespective of the load ratio and contaminants) was approximately 48% when extracted by EDTA and 46% when extracted by citric acid. The soil samples treated with ES2 exhibited average maximum removal efficiencies of 19% and 10% when extracted by EDTA and citric acid, respectively. It was observed that ES2 precipitated a maximum amount of calcium carbonate (CaCO3) when compared to ES1 and ES3 and retained the maximum amount of heavy metals in the soil by forming a CaCO3 shield on the heavy metals, thus decreasing their mobility. An approximate improvement of 30% in the retention of heavy metal ions was observed in soils treated with ESs when compared to untreated soil samples. Therefore, the study suggests that ESs can be an effective alternative in the remediation of soils contaminated with heavy metal ions.

Inhibition of Cd accumulation in winter wheat (Triticum aestivum L.) grown in alkaline soil using mercapto-modified attapulgite

Cd contamination in agricultural soils threatens the safety of agricultural products and poses human health risk via food chain. However, the remediation of Cd polluted alkaline soils has not drawn the public concern, and the corresponding efficient amendments that can reduce Cd accumulation in crop grains are relatively few. In current study, mercapto-modified attapulgite (MA in abbreviation) was selected as the amendment to conduct winter wheat (Triticum aestivum L.) cultivation pot experiment to investigate the effect of MA on Cd accumulation in winter wheat and Cd bioavailability in alkaline soil. MA had no adverse impact on the normal growth of winter wheat but could inhibit Cd accumulation in wheat grain of both cultivars grown in alkaline soil with a maximum reduction of 75%, while pH-regulating amendment sepiolite had no reduction effect. In the term of soil chemistry, MA could decrease the zeta potential of soil particles and enhance the sorption amount of Cd on soil particles, resulted in the increase of Fe-Mn-oxides bounded Cd fraction in alkaline soil. The enhanced sorption effect combined with complexation effect of MA itself, made the exchangeable and bioavailable Cd concentrations in the soil decrease. In the term of plant uptake, MA could inhibit the uptake of Cd via roots from the soil, and hinder Cd transfer from roots to grains. MA had environmental friendliness and capability in the aspect of soil pH, effective cation exchange capacity and available micronutrients in the soil. The high performance of MA in inhabitation of Cd in winter wheat revealed that it was an efficient immobilization agent with great application potential for Cd-polluted alkaline soil.

Soil Cd, Cr, Cu, Ni, Pb and Zn sorption and retention models using SVM: Variable selection and competitive model

The aim of this study was to model the sorption and retention of Cd, Cu, Ni, Pb and Zn in soils. To that extent, the sorption and retention of these metals were studied and the soil characterization was performed separately. Multiple stepwise regression was used to produce multivariate models with linear techniques and with support vector machines, all of which included 15 explanatory variables characterizing soils. When the R-squared values are represented, two different groups are noticed. Cr, Cu and Pb sorption and retention show a higher R-squared; the most explanatory variables being humified organic matter, Al oxides and, in some cases, cation-exchange capacity (CEC). The other group of metals (Cd, Ni and Zn) shows a lower R-squared, and clays are the most explanatory variables, including a percentage of vermiculite and slime. In some cases, quartz, plagioclase or hematite percentages also show some explanatory capacity. Support Vector Machine (SVM) regression shows that the different models are not as regular as in multiple regression in terms of number of variables, the regression for nickel adsorption being the one with the highest number of variables in its optimal model. On the other hand, there are cases where the most explanatory variables are the same for two metals, as it happens with Cd and Cr adsorption. A similar adsorption mechanism is thus postulated. These patterns of the introduction of variables in the model allow us to create explainability sequences. Those which are the most similar to the selectivity sequences obtained by Covelo (2005) are Mn oxides in multiple regression and change capacity in SVM. Among all the variables, the only one that is explanatory for all the metals after applying the maximum parsimony principle is the percentage of sand in the retention process. In the competitive model arising from the aforementioned sequences, the most intense competitiveness for the adsorption and retention of different metals appears between Cr and Cd, Cu and Zn in multiple regression; and between Cr and Cd in SVM regression.

Risk element sorption/desorption characteristics of dry olive residue: a technique for the potential immobilization of risk elements in contaminated soils

Olive oil production is one of the most relevant agroindustrial activities in the Mediterranean region and generates a huge amount of both solid and semi-solid wastes, the uncontrolled disposal of which might lead to serious environmental problems. Due to its organic matter and mineral nutrient content, the waste material can be applied to agricultural soil as a fertilizer. However, due to its high organic matter content, dry olive residue (DOR), commonly called "alperujo," has the potential to immobilize risk elements in contaminated soils. The main objective of this study was to assess the possible effect of DOR on sorption of risk elements such as cadmium (Cd), lead (Pb), and zinc (Zn) in the soil. A set of batch sorption experiments were carried out to assess the ability of DOR to adsorb Cd, Pb, and Zn where the effect of the preceding biotransformation of DOR by four species of fungi: Penicillium chrysogenum, Coriolopsis floccosa, Bjerkhandera adusta, and Chondrostereum purpureum was compared. The Freundlich and Langmuir sorption isotherms were calculated to assess the sorption characteristics of both transformed and non-transformed DOR. The results showed good potential sorption capacity of DOR, especially for Pb and to a lesser extent for Cd and Zn. Better sorption characteristics were reported for the biotransformed DOR samples, which are expected to show higher humification of the organic matter. However, the desorption experiments showed weakness and instability of the DOR-bound elements, especially in the case of Zn. Thus, future research should aim to verify the DOR sorption pattern in contaminated soil as well as the potential stabilization of the DOR element bounds where the increase of the pH levels of the DOR samples needs to be taken into account.

Contributions of a compost-biochar mixture to the metal sorption capacity of a mine tailing

One technique applied to restore degraded or contaminated soils is to use amendments made of different types of waste materials, which in turn may contain metals such as Cu, Pb and Zn. For this reason, it is important to determine the capacity of the soil to retain these materials, and to compare the sorption capacity between an amended soil and another unamended soil. The aim of this study was to determine the mobility and availability of these metals in the soil after applying the amendment, and how it affected the soil's sorption capacity. Sorption isotherms were compared with the empirical models of Langmuir and Freundlich to estimate the sorption capacity. The overall capacity of the soils to sorb Cu, Pb or Zn was evaluated as the slope Kr. The amendments used in this study were a mixture made of compost and biochar in different proportions (20, 40, 60, 100 %), which were applied to the mine tailing from a settling pond from a copper mine. The mine tailing that were amended with the mixture of compost and biochar had a higher sorption capacity than the mine tailing from the unamended pond, and their sorption isotherms had a greater affinity towards Cu, Pb and Zn than the mine tailing that was studied. Therefore, the results obtained show that adding a mixture of compost and biochar favours the retention of Cu, Pb and Zn in mine tailing.

Physical distribution of Ni, Pb and Zn in reclaimed mine soils observed by FE-SEM with an EDS detector

The aim of this study was to physically demonstrate the associations between Ni, Pb and Zn and the different soil components. To achieve this, several soil samples were observed by field emission-scanning electron microscope (FE-SEM) equipped with an energy dispersive spectrometer (EDS) detector. The samples came from mine sites vegetated and/or amended with wastes (sewage sludges and paper mill residues). The concentrations of metals in the different soil fractions were quantified by a chemical sequential extraction in a previous study. The sorption capacity of the soils was evaluated with sorption experiments using the batch method. We corroborated the results obtained by the sequential extraction of metals as well as the sorption experiments with the observations from the FE-SEM with the EDS. We physically observed the associations between Ni, Pb and Zn and oxides, organic matter and clays. We also observed PbCaCO3 crystals in one of the soils, presumably formed during the sorption experiment. As it is not possible to know with complete certainty how Pb was retained by calcium in this soil by only using chemical methods, the use of microscopic techniques is crucial to ascertain how metals are associated with the different soil fractions.

Competitive sorption and desorption of trace elements by Tunisian Aridisols Calcorthids

The sorption and retention processes play an important role in determining the bioavaibility and fate of trace elements in soils. Sorption and desorption of Pb(2+), Zn(2+), Ni(2+), Cu(2+), and Co(2+) in three Tunisian Aridisols Calcorthids (AR1, AR2, and AR3) were studied using batch experiments. Sorption and retention capacities were determined by means of K r parameter and they were related to soil properties. The results showed that in all studied soils, K r values for Pb(2+) and Cu(2+) were higher than those of Zn(2+), Ni(2+), and Co(2+) indicating that soils have higher affinity for the first ones. The high sorption and retention capacity of the three studied soils is ascribed to their alkaline pH and their high carbonates contents favoring the precipitation of these elements. Moreover, bivariate correlation analysis showed that sorption and retention of the studied cations was also strongly correlated with clay fraction and Fe oxides contents. All soils show high sorption irreversibility of Pb(2+), Zn(2+), Ni(2+), Cu(2+), and Co(2+). The soils with highest sorption capacity show also the highest irreversibility.

Validation of TOF-SIMS and FE-SEM/EDS Techniques Combined with Sorption and Desorption Experiments to Check Competitive and Individual Pb2+ and Cd2+ Association with Components of B Soil Horizons

Sorption and desorption experiments were performed by the batch method on the B horizons of five natural soils: Umbric Cambisol, Endoleptic Luvisol, Mollic Umbrisol, Dystric Umbrisol, and Dystric Fluvisol. Individual and competitive sorption and desorption capacity and hysteresis were determined. The results showed that Pb²⁺ was sorbed and retained in a greater quantity than Cd²⁺ and that the hysteresis of the first was greater than that of the second. The most influential characteristics of the sorption and retention of Pb²⁺ were pH, ECEC, Fe and Mn oxides and clay contents. For Cd²⁺ they were mainly pH and, to a lesser extent, Mn oxides and clay content. The combined use of TOF-SIMS, FE-SEM/EDS and sorption and desorption analyses was suitable for achieving a better understanding of the interaction between soil components and the two heavy metals. They show the preferential association of Pb²⁺ with vermiculite, chlorite, Fe and Mn oxides, and of Cd²⁺ with the same components, although to a much lesser extent and intensity. This was due to the latter’s higher mobility as it competed unfavourably with the Pb²⁺ sorption sites. TOF-SIMS and FE-SEM/EDS techniques confirmed the results of the sorption experiments, and also provided valuable information on whether the soil components (individually or in association) retain Cd²⁺ and / or Pb²⁺; this could help to propose effective measures for the remediation of contaminated soils.

A review of the distribution coefficients of trace elements in soils: influence of sorption system, element characteristics, and soil colloidal properties

Knowledge about the behavior and reactions of separate soil components with trace elements (TEs) and their distribution coefficients (Kds) in soils is a key issue in assessing the mobility and retention of TEs. Thus, the fate of TEs and the toxic risk they pose depend crucially on their Kd in soil. This article reviews the Kd of TEs in soils as affected by the sorption system, element characteristics, and soil colloidal properties. The sorption mechanism, determining factors, favorable conditions, and competitive ions on the sorption and Kd of TEs are also discussed here. This review demonstrates that the Kd value of TEs does not only depend on inorganic and organic soil constituents, but also on the nature and characteristics of the elements involved as well as on their competition for sorption sites. The Kd value of TEs is mainly affected by individual or competitive sorption systems. Generally, the sorption in competitive systems is lower than in mono-metal sorption systems. More strongly sorbed elements, such as Pb and Cu, are less affected by competition than mobile elements, such as Cd, Ni, and Zn. The sorption preference exhibited by soils for elements over others may be due to: (i) the hydrolysis constant, (ii) the atomic weight, (iii) the ionic radius, and subsequently the hydrated radius, and (iv) its Misono softness value. Moreover, element concentrations in the test solution mainly affect the Kd values. Mostly, values of Kd decrease as the concentration of the included cation increases in the test solution. Additionally, the Kd of TEs is controlled by the sorption characteristics of soils, such as pH, clay minerals, soil organic matter, Fe and Mn oxides, and calcium carbonate. However, more research is required to verify the practical utilization of studying Kd of TEs in soils as a reliable indicator for assessing the remediation process of toxic metals in soils and waters.

Time of flight secondary ion mass spectrometry and high-resolution transmission electron microscopy/energy dispersive spectroscopy: a preliminary study of the distribution of Cu2+ and Cu2+/Pb2+ on a Bt horizon surfaces

Relatively new techniques can help in determining the occurrence of mineral species and the distribution of contaminants on soil surfaces such as natural minerals and organic matter. The Bt horizon from an Endoleptic Luvisol was chosen because of its well-known sorption capability. The samples were contaminated with Cu(2+) and/or Pb(2+) and both sorption and desorption experiments were performed. The preferential distribution of the contaminant species ((63)Cu and (208)Pb) to the main soil components and their associations were studied together with the effectiveness of the surface sorption and desorption processes. The results obtained were compared with non-contaminated samples as well as with previous results obtained by different analytical techniques and advanced statistical analysis. Pb(2+) competes favorably for the sorption sites in this soil, mainly in oxides and the clay fraction. Cu(2+) and Pb(2+) were mainly associated with hematite, gibbsite, vermiculite and chlorite. This study will serve as a basis for further scientific research on the soil retention of heavy metals. New techniques such as spectroscopic imaging and transmission electron microscopy make it possible to check which soil components retain heavy metals, thereby contributing to propose effective measures for the remediation of contaminated soil.

Influence of soil properties on the sorption and retention of cadmium, copper and lead, separately and together, by 20 soil horizons: comparison of linear regression and tree regression analyses

In this paper we compare linear regression with tree regression for analysis of the influence of soil properties on the sorption and retention of added Cd, Cu and Pb by 20 soil horizons typical of cropped soils in Galicia (N.W. Spain); our measure of sorption/retention capacity was K(r), a recently introduced adimensional parameter. Sorption and retention of Cd was depressed by the presence of Cu and Pb. The soil parameters that were most associated, overall, with differences in Cd, Cu and Pb sorption and retention were cation exchange capacity (CEC), pH and Mn oxides' content. Tree regression, which can take into account variation on both global and local scales, afforded better-fitting models than linear regression, which only reflects global tendencies; but for coherent interpretability of tree regression results it is just as important to avoid overfitting as in the case of linear regression.

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.