Reaction-based model describing competitive sorption and transport of Cd, Zn, and Ni in an acidic soil

Modeling the competitive sorption and transport of Ni(II) and Zn(II) in soils: Comparing two multicomponent approaches

The mobility of contaminants in soil is controlled by sorption reactions which can be affected by the presence of other solutes that compete for sorption sites. The ability to model such effects is necessary for evaluating the environmental risk of a given contaminant. In this study, the competitive sorption and transport of nickel (Ni) and zinc (Zn) in Olivier and Windsor soils was investigated using batch equilibration and miscible displacement experiments. During batch experiments, the sorption of Ni and Zn was mutually reduced in multicomponent systems, indicating that the metal cations compete for sorption sites. When applied concurrently, the retardation of both ions decreased and peak effluent concentrations increased relative to single ion experiments, demonstrating that competition increased the mobility of both ions during miscible displacement experiments. A novel Freundlich-type multicomponent isotherm (CDI) and its kinetic analog (CDIT) were developed and compared to the commonly used SRS isotherm and SRS-based kinetic approach (SRST) in describing the experimental data. The CDI provided a superior description of the competitive batch data, especially at low surface coverage, and may therefore be more applicable to multicomponent sorption than the SRS. The Olivier and Windsor transport data were best described by the CDIT and SRST, respectively, however, both models generally described the data well. Since both approaches gave comparable descriptions of the transport data while the CDI outperformed the SRS in describing the batch data, the CDI/CDIT may be more generally applicable to multicomponent systems and warrants further study.

Temporal evolution of surface and sub-surface geochemistry and microbial communities of Pb-rich mine tailings during phytostabilization: A one-year pilot-scale study

Old mine waste repositories can present health and/or environmental issues linked to their erosion, inducing dissemination of metals and metalloids in air and water that can be attenuated through phytostabilization. Here, the effect of this widespread phytomanagement option on the biogeochemistry of a Pb-rich mine waste was evaluated with a laboratory pilot-scale experiment giving access to the non-saturated and saturated zones below the rhizosphere compartment. Amendment of the tailings surface with biochar, manure and iron-oxide-rich ochre promoted growth of the seeded Agrostis capillaris plants. These events were accompanied by an increase of pH and a decrease of Pb concentration in pore water of the surface layer, and by a transient increase of Pb, Zn, and Ba concentrations in the deeper saturated levels. Macroscopic and microscopic observations (SEM) suggest that Pb was immobilized in A. capillaris rhizosphere through mechanical entrapment of tailing particles. Microbial taxonomic and metabolic diversities increased in the amended phytostabilized surface levels, with a rise of the proportion of heterotrophic micro-organisms. Below the surface, a transient modification of microbial communities was observed in the non-saturated and saturated levels, however 11 months after seeding, the prokaryotic community of the deepest saturated zone was close to that of the initial tailings. pH and water saturation seemed to be the main parameters driving prokaryotic communities' structures. Results obtained at pilot-scale will help to precisely evaluate the impacts of phytostabilization on the temporal evolution of reactions driving the fate of pollutants inside the tailings dumps.

Characteristics of the immobilization process of arsenic depending on the size fraction released from excavated rock/sediment after the addition of immobilization materials

Chemical immobilization is an effective technique to suppress the release of arsenic from naturally arsenic-containing excavated rock/sediment. For designing the chemical immobilization technique, it is important to understand that the immobilization of arsenic depends on the sizes of ionic arsenic and arsenic retained on the colloids and suspended particles that are released from the excavated rock/sediment. Tests on the size fractionation of the arsenic released and the subsequent immobilization were conducted. The total amount of the size fraction of arsenic released from six excavated rock/sediment ranged from 0.16 to 0.75 mg kg^-1. The distributions of size fraction of arsenic released were categorized into three types: the dominant fraction was suspended particle fraction (SP-F) and ionic fraction (I-F), and a compatible amount of SP-F and I-F was included. Steel slag, calcium oxide, and ferrihydrite, which can effectively and stably immobilize ionic arsenic with different mechanisms, decreased the total amounts of the size fraction of arsenic released at 28%-84%, 59%-83%, and 57%-84%, respectively. Ferrihydrite and calcium oxide greatly reduced the I-F and the small and large colloid fractions. The steel slag was effective in reducing the SP-F at >86 %. In most arsenic fractions, the immobilized arsenic was not re-released at <7 %. This study provides the first experimental evidence of the variation in the released arsenic size depending on the excavated rock/sediment. In addition, the size fraction of the arsenic that could be immobilized depended on the immobilizing material. Thus, it is suggested that the combined application of immobilization materials would present a useful approach for immobilizing various released arsenic phases and preventing immobilized arsenic from re-release.

Co-transport of heavy metals in layered saturated soil: Characteristics and simulation

Interest in soil pollution by multiple heavy metals has been growing over the last decades. However, few experiments combining numerical analyses with solute transport in layered soil can be found in the literature. Here, the retention and fate of three coexisting metal ions, Cu, Cd, and Zn, in layered soils were investigated to evaluate soil co-contamination through batch and column experiments. Results showed high amounts of Cu adsorption and retention by soils, followed by Cd and Zn. The partial concentration of Zn in effluent was greater than the input from competition adsorption and the 'snow plow effect'. These findings indicate the high potential risk of Zn and Cd groundwater pollution when Cu, Cd, and Zn co-exist in the soil. Adsorption isotherms obtained from batch experiments were well described by Freundlich equation. Breakthrough curves (BTCs) obtained from column experiments were well described by standard convection-dispersion equation (CDE) for Br, and Tow-site (TSM) and One-site models (OSM) for metals except for Zn, using the Levenberg-Marquardt nonlinear optimization algorithm. However, the parameters were poorly constrained by the available observational data due to high correlation between parameters, rather than insensitivity to model outputs. The Generalized Likelihood Uncertainty Estimation (GLUE) method did not only qualify the uncertainty of parameters for solute transport in layered medium, but estimate prediction uncertainty. Prediction bounds basically captured the observed Br, Zn and Cd BTCs, while systematically overestimated the effluent Cu concentration. Comparing with the optimization, GLUE method can improve prediction reliability of heavy metal transport in layered soils.

Selective adsorption of uranyl and potentially toxic metal ions at the core-shell MFe2O4-TiO2 (M=Mn, Fe, Zn, Co, or Ni) nanoparticles

Potentially toxic metal ions (Xⁿ⁺: Rb⁺, Sr²⁺, Cr³⁺, Mn²⁺, Ni²⁺, Zn²⁺, Cd²⁺) usually coexist with uranyl (UO₂⁺), which will have a great influence on the selective adsorption process. Here, the core-shell MFe₂O₄-TiO₂ (M = Mn, Fe, Zn, Co, or Ni) nanoparticles were synthesized and assessed as new selective adsorbents. The results reveal that TiO₂(101) preferentially grows along the MFe₂O₄(311)/(111) orientation. The M²⁺ ions as the mediators transfer the holes from MFe₂O₄ to TiO₂, at the conduction bands. On the TiO₂(101) surfaces and TiO₂(101)-TiO₂(101) gaps, the paired active electrons mainly complex with water molecules as hydroxyl radicals to capture Xⁿ⁺ ions, forming an ion layer to block UO₂²⁺ from being adsorbed. Simultaneously, it should be noted that an interesting adsorption pathway was UO₂²⁺ being horizontally and irreversibly adsorbed in the MFe₂O₄(311)/(111)-TiO₂(101) interface, and therein, the stable adsorption capacity was found to be 66.78 mg g^-1 in the MnFe₂O₄(311)/(111)-TiO₂(101) interface. Finally, a mechanism of hybrid orbitals between MnFe₂O₄-TiO₂ and UO₂⁺-Xⁿ⁺ was proposed.

A large-scale field trial experiment to derive effective release of heavy metals from incineration bottom ashes during construction in land reclamation

Recycling of incineration bottom ashes (IBA) is attracting great interest as it is considered as a vital aspect for closing the waste loop to achieve sustainable development at the growing cities around the world. Various laboratory-testing methods are developed to assess the release potential of heavy metals - one of the most important concerns of using IBA, by reflecting the release conditions of heavy metals from IBA based on the targeted land reclamation application scenarios and corresponding environmental conditions. However, realistic release of the concerned elements in actual application with the presence of complex environment could possibly deviate from the outcomes produced by leaching tests carried out in the laboratory. Hence, a set of large-scale column trial experiments was performed to experimentally determine the effective release of heavy metals, when IBA is used as a filling material in land reclamation. 20 tons of IBA and 320 m³ of seawater were used in six column trial experiments. The release of 13 heavy metal elements was analyzed through multiple aspects which included kinetics of release, distribution of elements in seawater and the impacts of two different dumping methods, with and without application of a chute. After dumping of IBA into the seawater, almost instantaneous release of heavy metals with uniform horizontal dispersion was observed. Higher concentration of these elements was observed near the bottom of the column, especially when a chute was applied. Comparative analysis was then carried out to establish relationships between the results obtained from the column trial with batch leaching test carried out in the laboratory. Distinctive relationships were observed for different heavy metals which suggests the need of pursuance of further understanding on leaching of IBA in real application scenario and complex environment.

Insight into the Synergistic Effect on Selective Adsorption for Heavy Metal Ions by a Polypyrrole/TiO2 Composite

Polymer/metal oxide composites are promising candidates for the treatment of water pollution. Adsorption selectivity as well as a large adsorption capacity are two key factors for treating wastewater containing multiple ions. Herein, a PPy⁺/TiO₂(O^-) composite with a heterojunction structure was first discovered to have novel selectivity toward heavy metal ions. An interesting self-doping nature of TiO₂(O^-) together with SO₄^2- for PPy⁺ was reported. This interesting structure contributed to an impressive selective adsorption capability with an ascending order of Zn²⁺ > Pb²⁺ ≫ Cu²⁺ in a ternary ion system, where the adsorption for Cu²⁺ could be almost suppressed. Through the designed adsorption experiments and characterization techniques including Fourier transform infrared, thermogravimetric analysis, and X-ray photoelectron spectroscopy, a universal synergistic mechanism for PPy⁺/TiO₂(O^-) composite was first proposed and confirmed. The doping and dedoping of metal oxide (dopant) from the polymer dictates the adsorption selectivity, where the selectivity is determined by the interaction between TiO₂ and heavy metal ions. This work may provide some useful guidelines for designing adsorbents with selectivity toward specific heavy metal ions.

The effect of simulated acid rain on the stabilization of cadmium in contaminated agricultural soils treated with stabilizing agents

Stabilization technology is one of widely used remediation technologies for cadmium (Cd)-contaminated agricultural soils, but stabilized Cd in soil may be activated again when external conditions such as acid rain occurred. Therefore, it is necessary to study the effect of acid rain on the performance of different stabilizing agents on Cd-polluted agriculture soils. In this study, Cd-contaminated soils were treated with mono-calcium phosphate (MCP), mono-ammonium phosphate (MAP), and artificial zeolite (AZ) respectively and incubated 3 months. These treatments were followed by two types of simulated acid rain (sulfuric acid rain and mixed acid rain) with three levels of acidity (pH = 3.0, 4.0, and 5.6). The chemical forms of Cd in the soils were determined by Tessier's sequential extraction procedure, and the leaching toxicities of Cd in the soils were assessed by toxicity characteristic leaching procedure (TCLP). The results show that the three stabilizing agents could decrease the mobility of Cd in soil to some degree with or without simulated acid rain (SAR) treatment. The stabilization performances followed the order of AZ < MAP < MCP. Acid rain soaking promoted the activation of Cd in stabilized soil, and both anion composition and pH of acid rain were two important factors that influenced the stabilization effect of Cd.

Evaluation of environmental and anthropogenic influences on ambient background metal and metalloid concentrations in soil

There has been a global shift in environmental risk assessment towards quantifying ambient background concentrations of metals/metalloids in soil. Whilst bedrock has been shown to be a key driver of metal/metalloid variability in soil, few researchers have assessed controls of ambient background concentrations in soils of similar bedrock. A soil survey was undertaken ofGreater Melbourne, Greater Geelong, Ballarat and Mitchell in Victoria, Australia for elements of potential environmental concern: As, Cd, Cr, Cu, Ni, Hg, Pb and Zn. Samples (n=622) were collected from surface (0 to 0.1m) and sub-surface (0.3 to 0.6m) soils, overlying Tertiary-Quaternary basalt, Tertiary sediments and Silurian siltstone and sandstone. In addition, background soil data from open-source environmental assessment reports (n=5512) were collated to support the understanding of natural enrichment, particularly at depths >0.6m. Factor analysis, supported by correlation analysis and auxiliary geo-spatial data, provided an improved understanding of where and how background metal/metalloid enrichment occurs in the environment. Weathering during paleoclimates was the predominant influence of background metal/metalloid variability in soils overlying similar bedrock. Other key influences of metal/metalloid variability in soil included hydraulic leaching of alkali elements, biological cycling, topography and alluvial transfer of silt and sand from mineralised regions. In addition, urbanisation positively correlated with Pb and Zn concentrations in surface soils suggesting that anthropogenic activities may have resulted in diffuse Pb and Zn contamination of urban soil.

Statistical comparison of leaching behavior of incineration bottom ash using seawater and deionized water: Significant findings based on several leaching methods

Bottom ashes generated from municipal solid waste incineration have gained increasing popularity as alternative construction materials, however, they contains elevated heavy metals posing a challenge for its free usage. Different leaching methods are developed to quantify leaching potential of incineration bottom ashes meanwhile guide its environmentally friendly application. Yet, there are diverse IBA applications while the in situ environment is always complicated, challenging its legislation. In this study, leaching tests were conveyed using batch and column leaching methods with seawater as opposed to deionized water, to unveil the metal leaching potential of IBA subjected to salty environment, which is commonly encountered when using IBA in land reclamation yet not well understood. Statistical analysis for different leaching methods suggested disparate performance between seawater and deionized water primarily ascribed to ionic strength. Impacts of leachant are metal-specific dependent on leaching methods and have a function of intrinsic characteristics of incineration bottom ashes. Leaching performances were further compared on additional perspectives, e.g. leaching approach and liquid to solid ratio, indicating sophisticated leaching potentials dominated by combined geochemistry. It is necessary to develop application-oriented leaching methods with corresponding leaching criteria to preclude discriminations between different applications, e.g., terrestrial applications vs. land reclamation.

Mobility and Fate of Pollutants in the Aquifer System of the Northwestern Suez Gulf, Egypt

The northwestern part of Suez Gulf region is a strategic area in Egypt. It includes important sources of national income. To achieve the development goals, the government has established huge projects in this area (e.g. establishment and expanding of a large commercial port at Ain Sokhna, many industrial zones as well as tourism projects). The utilization of the Suez Gulf resources and their continuing development mainly depend on the creation of actual pollution control programs. The environmental quality control and pollution reduction activities are important ingredients of any economic development program. These different activities in this area depend mainly on the groundwater that is pumped intensively from different water bearing formations or aquifers. The main objective of the present work is compiling the previous studies from the 1980s up to 2015. These studies are concerned with estimating the concentrations of different pollutants in various ecosystems in the northwestern Suez Gulf region. Also, to provide an explanation for the movement of different pollutants such as organic and heavy metals from contaminated land to ground and surface (Gulf) waters. This issue has not been extensively surveyed before, and this review, gives specific directions for future monitoring and remediation strategies in this region.

Experimental determination of nonequilibrium transport parameters reflecting the competitive sorption between Cu and Pb in slag-sand column

Competitive sorption and resulting nonequilibrium transport of Cu and Pb were investigated using slag as a primary sorbent. A series of estimation models were applied based on the equilibrium, and nonequilibrium sorption respectively, and finally calibrated by incorporating the experimentally determined batch kinetic data. When applied individually, the behavior of metals in slag-sand column were well predicted by both equilibrium and nonequilibrium models in CXTFIT code. However, coexisting Cu and Pb exhibited competition for sorption sites, generating an irregular breakthrough curves such as overshoot (higher concentration in effluent than the feed concentration) of Cu and corresponding earlier peak of Pb followed by gradual re-rising. Although two-site nonequilibrium model further considers coupled hydrochemical process, desorption of the Cu from competition made the model prediction inaccurate. However, the parameter estimation could be improved by incorporating the experimentally determined mass transfer rate, ωexp from batch kinetics. Based on the calibrated model, the fraction of instantaneous retardation, βexp of Pb decreased from 0.41 in the single system to 0.30 in the binary system, indicating the shift from equilibrium to nonequilibrium state, where which of Cu increased from 0.39 to 0.94, representing the shift towards equilibrium. The modified results were also compared with five-step sequential extraction data, confirming that the shift of particular metal fractions from the competition triggered the nonequilibrium transport.

Adsorption of heavy metal from aqueous solution by dehydrated root powder of long-root Eichhornia crassipes

The root powder of long-root Eichhornia crassipes, as a new kind of biodegradable adsorbent, has been tested for aqueous adsorption of Pb, Zn, Cu, and Cd. From FT-IR, we found that the absorption peaks of phosphorous compounds, carbonyl, and nitrogenous compounds displayed obvious changes before and after adsorption which illustrated that plant characteristics may play a role in binding with metals. Surface properties and morphology of the root powders have been characterized by means of SEM and BET. Energy spectrum analysis showed that the metals were adsorbed on root powders after adsorption. Then, optimum quantity of powder, pH values, and metal ion concentrations in single-system and multi-system were detected to discuss the characteristics and mechanisms of metal adsorption. Freundlich model and the second-order kinetics equation could well describe the adsorption of heavy metals in single-metal system. The adsorption of Pb, Zn, and Cd in the multi-metal system decreased with the concentration increased. At last, competitive adsorption of every two metals on root powder proved that Cu and Pb had suppressed the adsorption performance of Cd and Zn.

Health risk assessment of heavy metals contamination in tomato and green pepper plants grown in soils amended with phosphogypsum waste materials

Phosphogypsum (PG) is a waste produced by the phosphate fertilizer industry that has relatively high concentrations of some heavy metals (e.g., Cd, Cr, Cu, Pb, V, and Zn). The present study was conducted to investigate heavy metal contamination in soils and vegetables (tomatoes and green peppers) and to evaluate the possible health risks associated with the consumption of vegetables grown in PG-amended soils. The enrichment factor values indicated that Pb, Cr, Cu, Ni, Zn, and V were depleted to minimally enriched, and Cd was moderately enriched. The pollution load index values indicated that the PG-amended soils were strongly polluted with Cd, moderately polluted with Cr and Ni, and slightly polluted with Pb, Cu, Zn and V. The geo-accumulation index values indicated that the PG-amended soils were uncontaminated with Pb, Cr, Cu, Ni, Zn, V, and moderately contaminated with Cd. The trace metal transfer for Cd, Cr, Pb, and Zn concentrations was below what are considered as acceptable limits (<1) for food production in soil and vegetables (tomatoes and green peppers) at each site area. Soil-to-plant transfer factor values decreased in order of Zn > Pb > Cd > Cr. The biological absorption coefficients in plants are, in order of highest to lowest, Pb > Zn > Cd > Cr, which suggests that Pb is more bioavailable to plants than Cd, Cr, and Zn. Furthermore, this study highlights that both adults and children consuming vegetables (e.g., tomatoes and green peppers) grown in PG-amended soils ingest significant amounts of the metals studied. However, the daily intake of metals (DIM) and the health risk index (HRI) values are <1, indicating a relative absence of health risks associated with the consumption of vegetables/fruits grown in PG-amended soils. However, while DIM and HRI values suggest that the consumption of plants grown in PG-amended soils is nearly free of risks, there are other sources of metal exposures such as dust inhalation, dermal contact, and ingestion (for children) of metal-contaminated soils, which were not included in this study.

The use of assemblage models to describe trace element partitioning, speciation, and fate: a review

The fate of trace elements in soils, sediments, and surface waters is largely determined by their binding to reactive components, of which organic matter, metal oxides, and clays are considered most important. Assemblage models, combining separate mechanistic complexation models for each of the reactive components, can be used to predict the solid-solution partitioning and speciation of trace elements in natural environments. In the present review, the authors provide a short overview of advanced ion-binding models for organic matter and oxides and of their application to artificial and natural assemblages. Modeling of artificial assemblages of mineral components and organic matter indicates that the interactions between organic and mineral components are important for trace element binding, particularly for oxyanions. The modeling of solid-solution partitioning in natural systems is generally adequate for metal cations but less so for oxyanions, probably because of the neglect of organic matter-oxide interactions in most assemblage models. The characterization of natural assemblages in terms of their components (active organic matter, reactive oxide surface) is key to successful model applications. Improved methods for characterization of reactive components in situ will enhance the applicability of assemblage models. Collection of compositional data for soil and water archetypes, or the development of relationships to estimate compositions from geospatially available data, will further facilitate assemblage model use for predictive purposes.

Aging effect on Zn retention on a calcareous soil: column experiments and synchrotron X-ray micro-spectroscopic investigation

In this study, a combination of column experiments and micro-analytical techniques exploiting synchrotron generated X-rays was used to assess the effect of aging time on Zn retention and mobility in the specific case of calcareous soils (high pH value, ≈ 8). The samples were subjected to aging for 2, 6, 17, and 63 days. Freshly added Zn mainly existed as an exchangeable form, and this metal fraction decreased over time due to Zn redistribution to stronger binding sites. Thus, after aging for 63 days, 45% of Zn is remobilized from exchangeable sites to stronger binding sites. μ-XRF maps were used to find correlations among elements in the sample, and μ-XANES spectra were recorded to precise Zn speciation. These analyses evidenced an increasing partitioning of Zn from organic matter to iron oxy(hydr)oxides over time. The occurrence of hydrozincite is evidenced in all samples.

Modeling approaches of competitive sorption and transport of trace metals and metalloids in soils: a review

Competition among various heavy metal species for available adsorption sites on soil matrix surfaces can enhance the mobility of contaminants in the soil environment. Accurate predictions of the fate and behavior of heavy metals in soils and geologic media requires the understanding of the underlying competitive-sorption and transport processes. In this review, we present equilibrium and kinetic models for competitive heavy metal sorption and transport in soils. Several examples are summarized to illustrate the impact of competing ions on the reactivities and mobility of heavy metals in the soil-water environment. We demonstrate that equilibrium Freundlich approaches can be extended to account for competitive sorption of cations and anions with the incorporation of competition coefficients associated with each reaction. Furthermore, retention models of the multiple-reaction type including the two-site nonlinear equilibrium-kinetic models and the concurrent- and consecutive-multireaction models were modified to describe commonly observed time-dependent behaviors of heavy metals in soils. We also show that equilibrium Langmuir and kinetic second-order models can be extended to simulate the competitive sorption and transport in soils, although the use of such models is limited due to their simplifying assumptions. A major drawback of the empirically based Freundlich and Langmuir approaches is that their associated parameters are specific for each soil. Alternatively, geochemical models that are based on ion-exchange and surface-complexation concepts are capable of quantifying the competitive behavior of several chemical species under a wide range of environmental conditions. Such geochemical models, however, are incapable of describing the time-dependent sorption behavior of heavy metal ions in competitive systems. Further research is needed to develop a general-purpose model based on physical and chemical mechanisms governing competitive sorption in soils.

Inverse optimization of hydraulic, solute transport, and cation exchange parameters using HP1 and UCODE to simulate cation exchange

Reactive transport modeling is a powerful tool to evaluate systems with complex geochemical relations. However, parameters are not always directly measurable. This study represents one of the first attempts to obtain hydrologic, transport and geochemical parameters from an experimental dataset involving transient unsaturated water flow and solute transport, using an automatic inverse optimization (or calibration) algorithm. The data come from previously published, controlled laboratory experiments on the transport of major cations (Na, K, Mg, Ca) during water absorption into horizontal soil columns that were terminated at different times. Experimental data consisted of the depth profiles of water contents (θ), Cl concentrations, and total aqueous and sorbed concentrations of major cations. The dataset was used to optimize several parameters using the reactive transport model, HP1 and the generic optimization code, UCODE. Although the soil hydraulic and solute transport parameters were also optimized, the study focused mainly on the geochemical parameters because the soil columns were constructed from disturbed soil. The cation exchange capacity and the cation exchange coefficients for two exchange models (Gapon and Rothmund-Kornfeld) were optimized. The results suggest that both calibrated models satisfactorily described the experimental data, although the Rothmund-Kornfeld model fit was slightly better. However, information content and surface response analyses indicated that parameters of the Gapon model are well identifiable, whereas those of the Rothmund-Kornfeld model were strongly correlated. The calibrated geochemical parameters were validated using an independent dataset. In agreement with the identifiability analysis, the Gapon approach was better than the Rothmund-Kornfeld model at calculating the observed concentrations of major cations in the soil solution and on the exchange sites.

Retention of heavy metals by carboxyl functional groups of biochars in small arms range soil

Long-term effectiveness of biochar for heavy metal stabilization depends upon biochar's sorptive property and recalcitrance in soil. To understand the role of carboxyl functional groups on heavy metal stabilization, cottonseed hull biochar and flax shive steam-activated biochar having a low O/C ratio (0.04-0.06) and high fixed carbon content (~80% dry weight basis) were oxidized using concentrated H(2)SO(4)/HNO(3) and 30% HNO(3). Oxidized and unoxidized biochars were characterized for O/C ratio, total acidity, pH, moisture, ash, volatile matter, and fixed carbon contents, Brunauer-Emmett-Teller surface area, and attenuated total reflectance Fourier transform infrared spectral features. Characterized biochars were amended (2%, 5%, 10%, and 20% in grams of biochar per gram of soil) on a sandy, slightly acidic (pH 6.27) heavy metal contaminated small arms range soil fraction (<250 μm) having low total organic carbon (0.518%) and low cation exchange capacity (0.95 cmol(c) kg(-1)). Oxidized biochars rich in carboxyl functional groups exhibited significantly greater Pb, Cu, and Zn stabilization ability compared to unoxidized biochars, especially in pH 4.9 acetate buffer (standard solution for the toxicity characteristic leaching procedure). Oppositely, only oxidized biochars caused desorption of Sb, indicating a counteracting impact of carboxyl functional groups on the solubility of anions and cations. The results suggested that appropriate selection of biochar oxidant will produce recalcitrant biochars rich in carboxyl functional groups for a long-term heavy metal stabilization strategy in contaminated soils.

Time-dependent changes of zinc speciation in four soils contaminated with zincite or sphalerite

The long-term speciation of Zn in contaminated soils is strongly influenced by soil pH, clay, and organic matter content as well as Zn loading. In addition, the type of Zn-bearing contaminant entering the soil may influence the subsequent formation of pedogenic Zn species, but systematic studies on such effects are currently lacking. We therefore conducted a soil incubation study in which four soils, ranging from strongly acidic to calcareous, were spiked with 2000 mg/kg Zn using either ZnO (zincite) or ZnS (sphalerite) as the contamination source. The soils were incubated under aerated conditions in moist state for up to four years. The extractability and speciation of Zn were assessed after one, two, and four years using extractions with 0.01 M CaCl(2) and Zn K-edge X-ray absorption fine structure (XAFS) spectroscopy, respectively. After four years, more than 90% of the added ZnO were dissolved in all soils, with the fastest dissolution occurring in the acidic soils. Contamination with ZnO favored the formation of Zn-bearing layered double hydroxides (LDH), even in acidic soils, and to a lesser degree Zn-phyllosilicates and adsorbed Zn species. This was explained by locally elevated pH and high Zn concentrations around dissolving ZnO particles. Except for the calcareous soil, ZnS dissolved more slowly than ZnO, reaching only 26 to 75% of the added ZnS after four years. ZnS dissolved more slowly in the two acidic soils than in the near-neutral and the calcareous soil. Also, the resulting Zn speciation was markedly different between these two pairs of soils: Whereas Zn bound to hydroxy-interlayered clay minerals (HIM) and octahedrally coordinated Zn sorption complexes prevailed in the two acidic soils, Zn speciation in the neutral and the calcareous soil was dominated by Zn-LDH and tetrahedrally coordinated inner-sphere Zn complexes. Our results show that the type of Zn-bearing contaminant phase can have a significant influence on the formation of pedogenic Zn species in soils. Important factors include the rate of Zn release from the contaminant phases and effects of the contaminant phase on bulk soil properties and on local chemical conditions around weathering contaminant particles.

Reactivity of nickel in soils: evidence of retention kinetics

The bioavailability and mobility of nickel (Ni) is highly dependent on the mechanisms associated with Ni adsorption-desorption and its kinetics in soils. To examine the characteristics of Ni retention and release, kinetic sorption batch experiments were performed on three soils having different properties, followed by Ni desorption using successive dilutions. Sorption of Ni by all soils was highly nonlinear and strongly kinetic, where the rate of Ni retention was rapid initially and was followed by gradual or somewhat slow retention behavior with increasing reaction time. Desorption of Ni was strongly irreversible and hysteretic in nature, indicating lack of equilibrium retention and/or irreversible or slowly reversible processes. A sequential extraction procedure provided evidence that a significant amount of Ni was irreversibly adsorbed on all soils. A nonlinear multireaction model with equilibrium-kinetic-irreversible reaction sites successfully described the retention (adsorption) and subsequent release of Ni on the different soils. The model was also capable of predicting Ni desorption kinetics based on adsorption data sets only.

Monitoring heavy metal concentrations in leachates from a forest soil subjected to repeated applications of sewage sludge

The aim of the study was to establish whether the repeated application of sewage sludge to an acid forest soil (Dystric Cambisol) would lead to short-term groundwater contamination. Sludge was applied at four loading rates (0, 2.4, 17 and 60 Mg ha(-1)) in two consecutive years and leachates were analysed. Heavy metal inputs to soils at the lowest dose were below EC regulations but, at higher doses, limits for Zn, Cd, Cr and Ni were exceeded. Repeated application of sludge at 60 Mg ha(-1) resulted in significantly (P<0.05) higher concentrations of Zn, Cd, Cr and Ni in the leachates than with other treatments. The drinking water standards for Cd and Ni were surpassed in all treatments. Control plots were contaminated by groundwater flow despite the existence of buffer zones between plots. This complicated interpretation of the results, highlighting the importance of careful design of this type of experiment.

Conductivity stopped-flow study of the kinetics of solutes uptake by colloidal particles of synthetic resins

The uptake rates of acids (hydrochloric acid, sulfuric acid, perchloric acid and acetic acid) by colloidal solid particles (mean diameter 0.35 microm) of DEAE (diethylaminoethyl Sephadex) resin was studied by the means of the stopped flow technique equipped with a conductivity detection. This original experimental approach allows to avoid hydrodynamics perturbations during data acquisition. For the sake of comparison NaOH uptake or K(+)/H(+) exchange by sulfonic Dowex resin particles experiments have also been performed. As stated in the literature we observed that the uptake rate limiting step is the solute diffusion in the solution layer surrounding the particles. To fit the experimental conductivity versus time curves on the basis of solutes diffusion concepts a simple mathematical equation fitting perfectly well the experimental data is proposed. The mathematical modeling of kinetic data obtained in non-stationary diffusion proposed here can substitute to the empiric pseudo-first- and -second-order models often used in environmental research.

Sorption-desorption tests to assess the risk derived from metal contamination in mineral and organic soils

We obtained the sorption isotherms of Cd, Cu, Pb and Zn from seven soils with contrasting properties, including mineral and organic soils. The distribution coefficients (Kd) were determined from batch tests in a solution that simulated the soil solution cationic composition. The Kd values of the target metals varied greatly depending on soil type and initial metal concentration. Sorption isotherms were fitted to Freundlich and Langmuir models, and derived parameters were correlated to soil properties through the construction of a correlation matrix and application of Principal Component Analysis. The batch Kd showed a satisfactory agreement with the Kd obtained from field-contaminated soils. The sorption tests were complemented with the estimation of the reversibility of metal sorption by the application of an extraction test. The extraction yields, which did not relate to the initial metal concentration, depended on the metal-soil combination, and showed no correlation to batch Kd values. The risk derived from a contamination event was estimated through the quantification of a Retention Factor, defined as the ratio of the Kd versus the extraction yield. Results showed that this was an excellent index to highlight which metal and soil represent the most vulnerable scenarios after a contamination event.

Study of the chemistry of an acid soil column and of the corresponding leachates after the addition of an anaerobic municipal sludge

A column leaching study was carried out over a period of 77 days to determine the changes in the chemistry of an acid soil and of the corresponding leachates after the addition of an anaerobic sludge (equivalent to 69 Mg DW ha(-1)). By the end of the experiment, the addition of the sludge to the soil had induced an increase in soil pH (from pH 3.6-4.0 to pH 4.1-4.8), in spite of the pronounced decrease in pH detected in the leachates by day 18 of the experiment. The decrease in pH (down to pH 3.3) occurred at the same time as leachate SO(4) and Fe peaked. Once the acidification attributed to sulphide oxidation ceased, the "liming effect" of the sludge became evident and counteracted further proton production - such as that associated with oxidation of NH(4) - at least for the duration of the study. Concentrations of Zn, Cd, Ni, and to a lesser extent, Pb in leachates displayed pulses at the beginning of the experiment (first 12 days), whereas the concentration of Cu followed a more irregular pattern; the concentrations of these metals never surpassed the European threshold values for drinking water. In contrast, concentrations of NO(3), Mn, and Cr in leachates had increased by the end of the experiment - in parallel with an increase in dissolved organic C (DOC) - and surpassed the European threshold for drinking water. Mineralisation of native soil organic C (SOC) was enhanced by the addition of this N-rich residue, and the organic C mass balance at the end of the experiment was negative. Nitrogen mass balance, although positive, exhibited a loss of 77% of the N added to the system. The results obtained indicate that application of this sewage sludge to a soil with a pH<5, at the loading rate used here, and without liming (i.e., non fulfilment of the requirements of the present European Directive) may pose a risk in terms of groundwater contamination.

Experimental Zn(II) retention in a sandy loam soil by very small columns

The use of column experiments, usually performed to better approximate field conditions, may provide information that is not available from batch experiments. In such experiments heavy metals are often adsorbed until saturation followed by desorption experiments. When the affinity of the metal to soil is high, the retention factor (R) could be greater than thousands and the duration of experiments can become impractically long. In order to use reasonable laboratory time, the flow rate should be increased or the column size decreased. The increase in flow rate produces undesirable kinetic and dispersion effects, so we used very small soil columns (pore volume=0.31-0.70 ml) and relatively high flow rates (0.03-0.12 ml min(-1)) in studies of Zn(II) adsorption and retention in soils. Conservative tracer flow column experiments under saturation conditions were carried out to determine flow parameters for different flow rates. Column pore volume (V(p)), Peclet numbers (Pe) and longitudinal dispersion coefficients (D(L)) were determined from breakthrough curves. The effect of type of electrolyte and ionic strength on the Zn(II) retention onto soil was determined. The influence of flow rate and bed height on the retention coefficient and on the mass transfer zone was also studied. The effect of different influent Zn(II) concentrations on the R values obtained was analyzed. Freundlich parameters from column experiments were compared with batch ones. The leaching efficiency of different electrolytes, salts of weak organic acids and EDTA was also studied.

Competitive Cu and Cd sorption and transport in soils: a combined batch kinetics, column, and sequential extraction study

The competitive effect influenced the transport behavior of Cu and Cd contrastingly in soils, as illustrated by the experimental findings obtained from column, batch kinetics, and sequential extraction tests. Of particular interest, Cd transport behavior changed from nonequilibrium in a single-metal system to equilibrium in a binary-metal system, whereas Cu exhibited a slightly greater degree of nonequilibrium transport under competition. The equilibrium time of specific sorption (approximately 7 days) was found to be much longer than that of nonspecific sorption (approximately 30 min). While there was a competitive effect on nonspecific sorption for both Cu and Cd, the majority of rate-limited specific sorption of Cd on oxide and organic matter fractions (contributing to approximately 20% of total sorption) was dramatically displaced by Cu. Such a strong suppression of specific sorption of Cd bythe presence of Cu resulted in a shorter equilibrium time of overall sorption, which probably accounts for its equilibrium transport. In contrast, the competitive effect on rate-limited sorption and transport behavior of Cu was less significant. This study demonstrated a correlation between the competitive effect of Cu and Cd on their nonspecific and specific sorption and the corresponding significance of rate-limited sorption and nonequilibrium transport behavior.

Effect of the cationic composition of sorption solution on the quantification of sorption-desorption parameters of heavy metals in soils

We obtained the sorption isotherms of Cd, Cu, Pb and Zn in clay, clay saline and organic soils. The distribution coefficients (K(d)) were determined in 0.02 eq l(-1) CaCl(2) and in a solution that simulated the soil solution cationic composition. The K(d) values greatly varied with the composition of the sorption solution and the initial metal concentration. The sorption experiments were complemented with the quantification of the extractable metal, to estimate the reversibility of metal sorption. The extraction yields depended on the metal-soil combination, and the initial metal concentration, showing no correlation with previous K(d) values. The effect of the solution composition in mobility predictions was estimated through a Retention Factor, defined as the ratio of the K(d) versus the extraction yield. Results showed that risk was over- or underestimated using the CaCl(2) medium in soils with a markedly different soil solution composition.

Performance assessment of a zeolite treatment wall for removing Sr-90 from groundwater

Laboratory and modeling studies were conducted to assess the potential performance of a permeable reactive barrier constructed of a natural zeolite material at the West Valley Demonstration Project in western New York State. The results of laboratory column tests indicated that the barrier material would be effective at removing strontium from groundwater under natural gradient conditions. Two one-dimensional contaminant transport models were developed to interpret the data. A single-solute retardation factor model provided good agreement with the column test data, but time-consuming extraction and analysis of the zeolite material was required to parameterize the model. A preliminary six-solute model was also developed based on the assumption of competitive cation exchange as the primary removal mechanism. Both models yielded similar predictions of the long-term performance of the barrier, but the cation exchange model predicted higher effluent concentrations during the first 1000 pore volumes of operation. The cation exchange framework has several advantages, including the ability to calibrate the model using only data from column effluent samples, and the ability to account for site-specific differences in the groundwater cation composition. However, additional laboratory work is needed to develop a suitably robust model.

Formation and dissolution of single and mixed Zn and Ni precipitates in soil: evidence from column experiments and extended X-ray absorption fine structure spectroscopy

The stability and the formation and dissolution kinetics of mixed trace metal precipitates in soils are currently unknown. The objective of this study was to investigate slow sorption and release processes of Zn and Ni in a loamy soil using a combination of soil column experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy. To investigate slow sorption processes, the soil material was packed into columns and leached with 5400 pore volumes of 10(-2) M CaCl2 solutions containing either ZnCl2 (5.2 x 10(-5) M) or NiCl2 (5.2 x 10(-5) M) or both ZnCl2 and NiCl2 (5.2 x 10(-5) M each). The Zn and Ni concentrations in the column effluents were monitored. The metal breakthrough curves showed that slow sorption processes lead to metal retention, whereby Zn was more strongly retained than Ni. In the experiment with both Zn and Ni present, amounts of Zn and Ni similar to those in the experiments with either Zn or Ni alone were retained. Analysis of soil samples by EXAFS spectroscopy showed that layered double hydroxide (LDH)-type precipitates had formed in all columns and that a mixed ZnNi-LDH had formed in the presence of both Zn and Ni. The dissolution of those precipitates under acidic conditions was assessed by subsequent leaching of the columns with a 10(-2) M CaCl2 solution at pH 3.0 (approximately 3000 pore volumes). When only Zn was present, 95% of the retained Zn was leached at pH 3. In contrast, only 23% of the retained Ni was leached in experiments with Ni alone. When Zn and Ni were present, 90% of the retained Zn and 87% of the retained Ni were released upon acidification. EXAFS analysis revealed that the LDH phases in the Zn experiment and the Zn-Ni experiment had been completely dissolved, while the LDH phase formed in the Ni experiment was still present. The higher resistance of Ni-LDH against dissolution at low pH could also be shown in dissolution studies with synthetic Zn-LDH, Ni-LDH, and ZnNi-LDH. Our results suggest that the individual rates at which Zn and Ni cations enter into the LDH structure determine the composition of the mixed ZnNi-LDH precipitate, and that the LDH composition determines the rate at which the LDH phase dissolves under acidic conditions.

Lead and zinc bioavailability to Eisenia fetida after phosphorus amendment to repository soils

Four phosphorus forms were investigated as potential soil amendments to decrease the bioavailability of Pb and Zn in two repository soils to the earthworm, Eisenia fetida. Treatments were evaluated by examining differences in bioaccumulation factors between amended and non-amended soils. Triple super phosphate at 5000 mg P/kg decreased both Pb and Zn bioavailability in both soils. Rock phosphate at 5000 mg P/kg decreased Zn bioavailability, but not Pb bioavailability in both repository soils. Monocalcium phosphate and tricalcium phosphate at 5000 mg P/kg did not significantly decrease Pb or Zn bioavailability to earthworms in either repository soil. In order to optimize phosphorus amendments, additional phosphorus (up to 15,000 mg P/kg) and lowered pH were used in a series of tests. The combination of lowering the pH below 6.0 and increasing phosphorus concentrations caused complete mortality in all triple super phosphate amended soils and partial mortality in the highest rock phosphate amended soils. Results indicate that triple super phosphate and rock phosphate are viable soil amendments, but care should be taken when optimizing amendment quantity and pH so that adverse environmental effects are not a by-product.

Leaching of heavy metals from contaminated soils: an experimental and modeling study

In this paper, we characterize the leaching of heavy metals (Ni, Cu, Zn, Cd, and Pb) from eight contaminated soils over a wide range of pH (pH 0.4-12) using an original approach based on batch pH-static leaching experiments in combination with selective chemical extractions and geochemical modeling. The leached concentrations of the heavy metals are generally much lower than the total concentrations and show a strong pH dependency, resulting in "V-shaped" leaching curves with orders of magnitude changes in solution concentrations. The "multisurface" model used incorporates adsorption to dissolved and solid organic matter (NICA-Donnan), iron/aluminum (hydr)oxide (generalized two-layer model) and clay (Donnan model). These models were applied without modifications, and only the standard set of binding constants and parameters was used (i.e., without any fitting). The model predictions of heavy metal leaching are generally adequate and sometimes excellent. Results from speciation calculations are consistent with the well-recognized importance of organic matter as the dominant reactive solid phase in soils. The observed differences between soils with respect to element speciation in the solid phase correspond to the relative amounts of the reactive surfaces present in the soils. In the solution phase, complexes with dissolved organic matter (DOM) are predominant over most of the pH range. Free metal ions (Me2+) are generally the dominant species below pH 4. The combination of the experimental and modeling approach as used in this study is shown to be promising because it leads to a more fundamental understanding of the pH-dependent leaching processes in soils. The "multisurface" modeling approach, with the selected sorption models, is shown to be able to adequately predict the leaching of heavy metals from contaminated soils over a wide range of conditions, without any fitting of parameters.

Modeling metal binding to soils: the role of natural organic matter

The use of mechanistically based models to simulate the solution concentrations of heavy metals in soils is complicated by the presence of different sorbents that may bind metals. In this study, the binding of Zn, Pb, Cu, and Cd by 14 different Swedish soil samples was investigated. For 10 of the soils, it was found that the Stockholm Humic Model (SHM) was able to describe the acid-base characteristics, when using the concentrations of "active" humic substances and Al as fitting parameters. Two additional soils could be modeled when ion exchange to clay was also considered, using a component additivity approach. For dissolved Zn, Cd, Ca, and Mg reasonable model fits were produced when the metal-humic complexation parameters were identical for the 12 soils modeled. However, poor fits were obtained for Pb and Cu in Aquept B horizons. In two of the soil suspensions, the Lund A and Romfartuna Bhs, the calculated speciation agreed well with results obtained by using cation-exchange membranes. The results suggest that organic matter is an important sorbent for metals in many surface horizons of soils in temperate and boreal climates, and the necessity of properly accounting for the competition from Al in simulations of dissolved metal concentrations is stressed.

Heavy metal release from contaminated soils: comparison of column leaching and batch extraction results

Heavy metals in soils may adversely affect environmental quality. In this study, we investigated the release of Zn, Cd, Pb, and Cu from four contaminated soils by column leaching and single and sequential batch extractions. Homogeneously packed soil columns were leached with 67 mL/g 10(-2) M CaCl2 to investigate the exchangeable metal pool and subsequently with 1400 mL/g 10(-2) M CaCl2 adjusted to pH 3 to study the potential of metal release in response to soil acidification. In two noncalcareous soils (pH 5.7 and 5.1), exchange by Ca resulted in pronounced release peaks for Zn and Cd that were coupled to the exchange of Mg by Ca, and 40 to 70% of total Zn and Cd contents were rapidly mobilized. These amounts compared well with exchangeable pools determined in single and sequential batch extractions. In two soils with near-neutral pH, the effluent concentrations of Zn and Cd were several orders of magnitude lower and no pronounced elution peaks were observed. This behavior was also observed for Cu and Pb in all four soils. When the soils were leached at pH 3, the column effluent patterns reflected the coupling of CaCO3 dissolution (if present) and other proton buffering reactions, proton-induced metal release, and metal-specific readsorption within the soil column. Varying the flow rate by a factor of five had only minor effects on the release patterns. Overall, Ca exchange and subsequent acidification to pH 3 removed between 65 and 90% of total Zn, Cd, Pb, and Cu from the four contaminated soils.

Combining selective sequential extractions, X-ray absorption spectroscopy, and principal component analysis for quantitative zinc speciation in soil

Selective sequential extractions (SSE) and, more recently, X-ray absorption fine-structure IXAFS) spectroscopy have been used to characterize the speciation of metal contaminants in soils and sediments. However, both methods have specific limitations when multiple metal species coexist in soils and sediments. In this study, we tested a combined approach, in which XAFS spectra were collected after each of 6 SSE steps, and then analyzed by multishell fitting, principal component analysis (PCA) and linear combination fits (LCF), to determine the Zn speciation in a smelter-contaminated, strongly acidic soil. In the topsoil, Zn was predominately found in the smelter-emitted minerals franklinite (60%) and sphalerite (30%) and as aqueous or outer-sphere Zn2+ (10%). In the subsoil, aqueous or outer-sphere Zn2+ prevailed (55%), but 45% of Zn was incorporated by hydroxy-Al interlayers of phyllosilicates. Formation of such Zn-bearing hydroxy-interlayers, which has been observed here for the first time, may be an important mechanism to reduce the solubility of Zn in those soils, which are too acidic to retain Zn by formation of inner-sphere sorption complexes, layered double hydroxides or phyllosilicates. The stepwise removal of Zn fractions by SSE significantly improved the identification of species by XAFS and PCA and their subsequent quantification by LCF. While SSE alone provided excellent estimates of the amount of mobile Zn species, it failed to identify and quantify Zn associated with mineral phases because of nonspecific dissolution and the precipitation of Zn oxalate. The systematic combination of chemical extraction, spectroscopy, and advanced statistical analysis allowed us to identify and quantify both mobile and recalcitrant species with high reliability and precision.

Slow formation and dissolution of Zn precipitates in soil: a combined column-transport and XAFS study

Recent spectroscopic studies have demonstrated the formation of layered double hydroxides (LDH) and phyllosilicates upon sorption of Zn2+, Ni2+, and Co2+ to clay minerals and aluminum oxides at neutral to alkaline pH and at relatively high initial metal concentrations (>1 mM). The intention of the present study was to investigate whether such phases also form in soil under slightly acidic conditions and at lower metal concentrations. Columns packed with a loamy soil were percolated with aqueous solutions containing 0.1 or 0.2 mM Zn, Ni, Co, and Cd in a 10 mM CaCl2 background at pH 6.5. Metal breakthrough curves indicated a rapid initial sorption step, resulting in retarded breakthrough fronts, followed by further slow metal retention during the entire loading period of 42 days (7000 pore volumes). Total metal sorption and the contribution of slow sorption processes decreased in the order Zn > Ni > Co > Cd. Leaching the reacted soil with 10 mM CaCl2 at pH 6.5 remobilized 8% of the total retained Zn, 15% of Ni, 21% of Co, and 77% of Cd. Subsequent leaching with acidified influent (pH 3.0) remobilized most of the remaining metals. X-ray absorption fine-structure (XAFS) spectroscopy revealed that slow Zn sorption was due to the formation of a Zn-Al LDH precipitate. Although Ni, Co, and Cd concentrations were too low for XAFS analysis, their leaching patterns suggest that part of Ni and Co were also incorporated in solid phases, while most sorbed Cd was still present as exchangeable sorption complex after 42 days. A small but significant percentage of the sorbed metals (2-5%) remained in the soil, even after leaching with more than 3000 pore volumes at pH 3.0, which may suggest micropore diffusion or incorporation into more stable mineral phases.

Contribution of individual sorbents to the control of heavy metal activity in sandy soil

A multisurface model is used to evaluate the contribution of various sorption surfaces to the control of heavy metal activity in sandy soil samples at pH 3.7-6.1 with different sorbent contents. This multisurface model considers soil as a set of independent sorption surfaces, i.e. organic matter (NICA-Donnan), clay silicate (Donnan), and iron hydroxides (DDL, CD-MUSIC). The activities of Cu2+, Cd2+, Zn2+, Ni2+, and Pb2+ in equilibrium with the soil have been measured using a Donnan membrane technique. The metal activities predicted by the model agree with those measured reasonably well over a wide concentration range for all the metals of interest except for Pb. The modeling results suggest that soil organic matter is the most important sorbent that controls the activity of Cu2+, Cd2+, Zn2+, and Ni2+ in these sandy soils. When metal loading is high in comparison with soil organic matter content, the contribution of clay silicates to metal binding becomes more important. Adsorption to iron hydroxides is found not significant in these samples for Cu, Cd, Zn, and Ni. However, for Pb the model estimates strong adsorption on iron hydroxides. The model predicts that acidification will not only lead to increased solution concentrations but also to a shift toward more nonspecific cation-exchange type binding especially for the metals Cd, Zn, and Ni. Lowering the pH has led to a loss of 56% of Cd, 69% of Zn, and 66% of Ni during 16 years due to increased leaching.