Geochemical Speciation Model Research Articles

Leaching and geochemical modeling of asbestos-cement waste and mine asbestos

Asbestos-Containing Materials (ACMs) were widely used in the construction sector but, due to their harmful health effects, many countries have banned their use. ACMs are classified as hazardous and, in contact with water, produce potentially harmful leachates. The objective of this work was to determine the leaching behavior of 20 elements from two asbestos-cement materials and mine asbestos samples across the entire pH range and varying liquid-to-solid ratios (column tests). The pH-dependence tests showed consistent leaching patterns across the three materials. Geochemical speciation model (LeachXS) predictions were successful in most cases of the batch experiments and were improved by adjusting iron oxides concentration for some elements. Model predictions were successful for fewer elements in the column experiments. Depending on the pH, element release was controlled by respective solid phase dissolution, sorption onto iron oxides and substitution in ettringite. Some leaching concentrations exceeded the EU limits for granular non-hazardous waste landfills. Considering the strongly alkaline nature of monolithic asbestos-cement waste undergoing carbonation, we propose all three materials to be disposed of in non-hazardous waste landfills, according to EU legislation. A case study concluded that geochemical modeling of ACMs leaching is a useful tool in estimating element release under various environmental conditions.

Binding mechanisms of trivalent chromium on colloidal phases from ultramafic systems: Insights from batch experiments and XAS analysis

Understanding the availability and mobility of chromium is a key factor in gaining insight into the fate of chromium and thus essential for assessing the risk of contamination and developing effective remediation strategies. In the natural environment, such as ultramafic systems, which are natural pools of chromium, chromium occurs mostly in the trivalent state (Cr(III)), which is known to have a high affinity for the solid phases such as particles and colloids. However, since Cr(VI) is more toxic, the vast majority of experimental studies focus on its reduction to Cr(III), which is often considered to form (hydr)oxide precipitates due to its poor solubility. Furthermore, while the oxidation state of Cr dictates its geochemical behavior, its interaction with particles and colloids is rarely controlled or monitored. Therefore, the interaction mechanisms between Cr(III) and particles or colloids require further attention. In this study, ten mineral phases (goethite, hematite, pyrolusite, kaolinite, montmorillonite, chlorite, serpentine, fuchsite, amphibole, and chromite) and one organic phase (humic acid), relevant to chromium bearing and carrier phases in ultramafic soils, were investigated to explore their interaction with Cr(III) species. These phases were first characterized by various techniques (including XRD, XRF, SEM, and BET). Adsorption and desorption experiments of Cr(III) on these phases were then carried out, and the distribution of Cr oxidation states in both the aqueous and solid phases was carefully monitored using UV–vis spectroscopy, geochemical speciation modeling, and Cr K-edge XAS spectroscopy. With the exception of pyrolusite, where partial oxidation of Cr(III) to Cr(VI) occurred, Cr(III) was shown to persist as a single oxidation state in all experiments. Desorption analyses showed significantly low desorbed rates, which further confirmed the prevalence of Cr(III). Experimental sorption isotherm data were interpreted using the Langmuir model and its linearized form, which provides important thermodynamic characteristics of adsorbents related to their structural and surface features. This study will provide better constraints for the understanding and prediction of Cr behavior and fate in the environment, especially in ultramafic systems.

A comparison of characterisation and modelling approaches to predict dissolved metal concentrations in soils

Environmental context It is useful to know the concentration of ‘labile’, or chemically active, metal in soils because it can be used to predict metal solubility and environmental impact. Several methods for extracting the labile metal from soils have been proposed, and here we have tested two of these to see how well the resulting data can be used to model metal solubility. Such mixed approaches can be applied to different soil types with the potential to model metal solubility over large areas. Rationale Predicting terrestrial metal dynamics requires modelling of metal solubility in soils. Here, we test the ability of two geochemical speciation models that differ in complexity and data requirements (WHAM/Model VII and POSSMs), to predict metal solubility across a broad range of soil properties, using differing estimates of the labile soil metal concentration. Methodology Using a dataset of UK soils, we characterised basic properties including pH and the concentrations of humic substances, mineral oxides and metals. We estimated labile metal by extraction with 0.05 mol L−1 Na2H2EDTA and by multi-element isotopic dilution (E-value). Dissolved concentrations of Ni, Cu, Zn, Cd and Pb were estimated in 0.01 mol L−1 Ca(NO3)2 soil suspensions using the total metal ({M}total), the EDTA-extracted pool ({M}EDTA) and the E-value ({M}E) as alternative estimates of the chemically reactive metal concentration. Results Concentrations of {M}EDTA were highly correlated with values of {M}E, although some systematic overestimation was seen. Both WHAM/Model VII and POSSMs provided reasonable predictions when {M}EDTA or {M}E were used as input. WHAM/Model VII predictions were improved by fixing soil humic acid to a constant proportion of the soil organic matter, instead of the measured humic and fulvic acid concentrations. Discussion This work provides further evidence for the usefulness of speciation modelling for predicting soil metal solubility, and for the usefulness of EDTA-extracted metal as a surrogate for the labile metal pool. Predictions may be improved by more robust characterisation of the soil and porewater humic substance content and quality.

Mechanistic insights into the leaching and environmental safety of arsenic in ceramsite prepared from fly ash

Utilizing fly ash to prepare ceramsite is a promising way to immobilize heavy metals and recycle industrial solid waste. However, traditional preparation method of fly ash ceramsite has the disadvantages of large ignition loss. Therefore, the present study applied the pressure molding method to enhance solid content and improve the strength of ceramsite. The optimal preparation conditions of ceramsite were suggested as preheating at 450 °C for 25 min followed by sintering at 1050 °C for 30 min. Under such conditions, ceramsite with high compressive strength of 10.8 Mpa, bulk density of 878 kg m−3, and 1-h water absorption of 18.5% was fabricated, in compliance with Chinese standard (GB/T 1743.1–2010). The arsenic leaching concentration from the resulting product was considerably lower than Chinese standard (GB 5085.3–2007). Moreover, arsenic volatilization during ceramsite calcination was insignificant, and the vast majority of arsenic remained in resulting ceramsite. A geochemical speciation model developed for the multiple component system in ceramsite suggested that FeAsO4, Ca5(OH) (AsO4)3, and hydrous ferric oxide adsorption are the primary mechanisms retaining arsenic in ceramsite. Additionally, based on density functional theory calculations and biotoxicity test, the binding site of arsenic atom on mineral components and the environmental safety of ceramsite was determined and evaluated.

Impact of oxidation and carbonation on the release rates of iodine, selenium, technetium, and nitrogen from a cementitious waste form

Evaluation of the long-term retention mechanisms and potential release rates for the primary constituents of potential concern (COPCs) (i.e., Tc, I, Se, and nitrate) is necessary to determine if Cast Stone, a radioactive waste form, can meet performance objectives under near-surface disposal scenarios. Herein, a mineral and parameter set accounting for the solubility of I and Se in Cast Stone was developed based on pH-dependent and monolithic diffusion leaching test results, to extend a geochemical speciation model previously developed. The impact of oxidation and carbonation as environmental aging processes on the retention properties of Cast Stone for primary COPCs was systematically estimated. Physically, the effective diffusion coefficients of 4 COPCs in Cast Stone were increased after carbonation and/or oxidation, reflecting an increase in permeability to diffusion. Chemically, i) pH & pe conditions in the original Cast Stone were favorable for the stabilization of Tc, but not for I, Se, and N; ii) oxidation (with/without carbonation) of Cast Stone changed the pe & pH conditions to be detrimental for Tc stabilization; and iii) carbonation (with/without oxidation) of Cast Stone modified the pH & pe conditions to be beneficial for the stabilization of I (in system with Ag added) and Se.

Leaching and geochemical evaluation of oxyanion partitioning within an active coal ash management unit

Field monitoring combined with laboratory leaching characterization and geochemical speciation modeling were used to identify important geochemical parameters and controlling mechanisms for leaching of oxyanions from coal ash in a disposal site. The site consisted of dry stacked ash on top of a former surface impoundment. Constituents leached from the dry stacked ash served as sources for the soluble fractions of constituents in underlying impoundment ash. Weathering of field ash was evidenced by the identification of calcite and ettringite. Porewater concentrations of As, B, and V in the field pH range (9–11) and liquid-to-solid ratio (L/S) condition (∼0.6 L/kg-dry) were primarily controlled by Ca-bearing precipitates (Ca-arsenate, B-substituted carbonate solid solution, Ca-vanadate, and V-substituted ettringite solid solution). The leachable fraction of Mo as a function of pH was limited by the available content in the solid, with aqueous concentrations of Mo being a function of L/S. Leaching behavior of Cr and Se was sensitive to the redox changes (from suboxic to oxic conditions) during field ash collection and laboratory leaching tests, including oxidative dissolution of insoluble Cr(III)-oxides and conversion of Se(0) in the solid to dissolved Se(IV)/Se(VI). Oxidation resulted in increased Cr concentrations from < 0.00005 (in porewater) to 0.001–0.1 mg/L (in laboratory eluates). The identified geochemical parameters and leaching-controlling mechanisms provide the bases for understanding and estimating the partitioning of oxyanions in other disposal sites with alkaline and suboxic conditions. This case study also provides a typical example of laboratory test results interpretation and field leaching behavior estimation for other studies.

Application and uncertainty of a geochemical speciation model for predicting oxyanion leaching from coal fly ash under different controlling mechanisms

Three primary mechanisms (adsorption to iron oxides or analogous surfaces, co-precipitation with Ca, and substitution in ettringite) controlling oxyanion retention in coal fly ashes (CFAs) were identified by differentiating the leaching behavior of As, B, Cr, Mo, Se, and V from 30 CFAs. Fidelity evaluation of geochemical speciation modeling focused on six reference CFAs representing a range of CFA compositions, whereby different leaching-controlling mechanisms of oxyanions were systematically considered. For three reference CFAs with low Ca and S content, calibration of adsorption reactions for the diffuse double-layer model for hydrous ferric oxides improved the simultaneous prediction of oxyanion leaching, which reduced uncertainties in Se and V predictions caused by nonideal adsorption surfaces and competitive adsorption effects. For two reference CFAs with intermediate Ca content, the solubility constants for Ca-arsenates from literature and postulated phases of B, Cr, Se, and V were used to describe co-precipitation of oxyanions with Ca-bearing minerals under alkaline conditions. For the reference CFA with high Ca and S content, an ettringite solid solution was used to capture the simultaneous retention of all oxyanions at pH> 9.5. Overall, the simultaneous leaching predictions of oxyanions from a wide range of CFAs were improved by calibration of adsorption reactions and controlling solid phases.

Impact of carbonation on leaching of constituents from a cementitious waste form for treatment of low activity waste at the DOE Hanford site

Carbonation can be a major aging process during disposal of alkaline cementitious waste forms and can impact constituent leaching by changes in material alkalinity, pore structure, and controlling mineral phases. The effect of carbonation on the leaching of major and trace constituents from Cast Stone, a cementitious waste form developed to treat high salt content low activity waste, was studied through a combination of leaching experiments and reactive transport simulations. Diffusive transport of constituents in the waste form was evaluated using reactive transport modeling of diffusion-controlled leaching test results and a geochemical speciation model derived from pH-dependent leaching. Comparisons between Cast Stone materials aged under nitrogen, air, and 2% carbon dioxide in nitrogen showed that carbonation impacts solubility, physical retention and observed diffusivity of major and trace constituents. Carbonation under 2% CO2 decreased the diffusion-controlled leaching of chromium by two orders of magnitude. Modeling results suggest that carbonation may also decrease solubility of technetium while changes to microstructure by carbonation increases effective diffusivity of constituents in Cast Stone.

The influence of redox conditions on aqueous-solid partitioning of arsenic and selenium in a closed coal ash impoundment

A closed coal ash impoundment case study characterized the effects of field redox conditions on arsenic and selenium partitioning through monitoring of porewater and subsurface gas in conjunction with geochemical speciation modeling. When disposed coal ash materials and porewater were recovered for testing, oxidation led to lower arsenic and higher selenium concentrations in leaching test extracts compared to porewater measurements. Multiple lines of evidence suggest multiple mechanisms of arsenic retention are plausible and the concurrent presence of several redox processes and conditions (e.g., methanogenesis, sulfate reduction, and Fe(III)-reduction) controlled by spatial gradients and dis-equilibrium. Geochemical speciation modeling indicated that, under reducing field conditions, selenium was immobilized through the formation of insoluble precipitates Se(0) or FeSe while arsenic partitioning was affected by a progression of reactions including changes in arsenic speciation, reduction in adsorption due to dissolution and recrystallization of hydrous ferric oxides, and precipitation of arsenic sulfide minerals.

Algorithms for activity correction models for geochemical speciation and reactive transport modeling

AbstractReactive transport softwares are today one of the cornerstones of environmental research. They contain multiphysics with very complex algorithms, including flow, transport, chemical, and sometimes heat transport, mechanical, and/or biological algorithms. Because of this complexity, some parts of these algorithms still have not been sufficiently studied. In this work, we focus on algorithms for activity correction, a specific subset of equilibrium chemistry algorithms. We show that the most used algorithm (the inner fixed‐point algorithm) or the most rigorous algorithm (the full Newton) might not be the most efficient, and we propose a new one, the outer fixed‐point algorithm, which is more robust and faster than other algorithms.

Development of a Geochemical Speciation Model for Use in Evaluating Leaching from a Cementitious Radioactive Waste Form.

Cast Stone has been developed to immobilize a fraction of radioactive waste at the Hanford Site; however, constituents of potential concern (COPCs) can be released when in contact with water during disposal. Herein, a representative mineral and parameter set for geochemical speciation modeling was developed for Cast Stone aged in inert and oxic environments, to simulate leaching concentrations of major and trace constituents. The geochemical speciation model was verified using a monolithic diffusion model in conjunction with independent monolithic diffusion test results. Eskolaite (Cr2O3) was confirmed as the dominant mineral retaining Cr in Cast Stone doped with 0.1 or 0.2 wt % Cr. The immobilization of Tc as a primary COPC in Cast Stone was evaluated, and the redox states of porewater within monolithic Cast Stone indicated by Cr are insufficient for the reduction of Tc. However, redox states provided by blast furnace slag (BFS) within the interior of Cast Stone are capable of reducing Tc for immobilization, with the immobilization reaction rate postulated to be controlled by the diffusive migration of soluble Tc in porewater to the surface of reducing BFS particles. Aging in oxic conditions increased the flux of Cr and Tc from monolithic Cast Stone.

Drying model of a high salt content cementitious waste form: Effect of capillary forces and salt solution

A water transport model coupling capillary liquid flow with vapor diffusion is developed to describe the drying process for a cementitious waste form with high salinity porewater. Vapor-liquid equilibrium is formulated as the driving force for vapor diffusion and the model accounts for pore capillary and high salinity effects on water thermodynamic activity. Pore filling and porewater surface tension as a function of pore size distribution and water saturation have been quantified for the material. Geochemical speciation modeling is used to simulate porewater activity as a function of composition over the range of saturation. The theoretical relationship between relative humidity and water saturation generally agrees with experimental measurement, and the developed model is capable of predicting drying rates under various external relative humidity conditions. The model was developed to be incorporated into reactive transport models considering the effects of drying such as salt redistribution and efflorescence.

Transport and speciation of uranium in groundwater-surface water systems impacted by legacy milling operations

Growing worldwide concern over uranium contamination of groundwater resources has placed an emphasis on understanding uranium transport dynamics and potential toxicity in groundwater-surface water systems. In this study, we utilized novel in-situ sampling methods to establish the location and magnitude of contaminated groundwater entry into a receiving surface water environment, and to investigate the speciation and potential bioavailability of uranium in groundwater and surface water. Streambed temperature mapping successfully identified the location of groundwater entry to the Little Wind River, downgradient from the former Riverton uranium mill site, Wyoming, USA. Diffusive equilibrium in thin-film (DET) samplers further constrained the groundwater plume and established sediment pore water solute concentrations and patterns. In this system, evidence is presented for attenuation of uranium-rich groundwater in the shallow sediments where surface water and groundwater interaction occurs. Surface water grab and DET sampling successfully detected an increase in river uranium concentrations where the groundwater plume enters the Little Wind River; however, concentrations remained below environmental guideline levels. Uranium speciation was investigated using diffusive gradients in thin-film (DGT) samplers and geochemical speciation modelling. Together, these investigations indicate uranium may have limited bioavailability to organisms in the Little Wind River and, possibly, in other similar sites in the western U.S.A. This could be due to ion competition effects or the presence of non- or partially labile uranium complexes. Development of methods to establish the location of contaminated (uranium) groundwater entry to surface water environments, and the potential effects on ecosystems, is crucial to develop both site-specific and general conceptual models of uranium behavior and potential toxicity in affected ground and surface water environments.

A comprehensive probabilistic approach for integrating and separating natural variability and parametric uncertainty in the prediction of distribution coefficient of radionuclides in rivers

A geochemical speciation model was developed to predict Distribution coefficients (Kds) of radionuclides (RNs) in rivers. The model takes into account complexation of RNs with inorganic ligands, sorption of RNs with hydrous ferric oxides, complexation of RNs with dissolved and particulate organic carbon (DOC and POC) and sorption and/or co-precipitation of RNs to carbonates. A sorption model of Cs onto clay was also integrated. The tool is also designed to conduct uncertainty and sensitivity analysis. Sensitivity analysis follows a stepwise structured approach, starting from computationally ‘inexpensive’ Morris method to most costly variance-based EFAST method. A nested Monte Carlo approach was also implemented to separate natural variability and lack of knowledge in global uncertainty assessment. As case studies, Kd distributions were estimated for Co, Mn, Ag and Cs in seven French rivers. Uncertainty analysis allowed to quantify Kd ranges that can be expected when considering all the sensitive parameters together.

Bacterial predation limits microbial sulfate-reduction in a coastal acid sulfate soil (CASS) ecosystem

Microbial iron and sulfate reduction are the primary drivers of coastal acid sulfate soil (CASS) passive bioremediation schemes. Microbial sulfate reduction is the limiting step for pyrite formation, a desirable endpoint for CASS remediation. Little is known, however, about the impacts of microbial activity or species interaction on long-term iron and sulfur cycling in CASS ecosystems. Using a combination of molecular biology, geochemical speciation and artificial intelligence-powered computational modelling, we deduced from microbial activity patterns (RNA-based) and geochemical measurements a best-fit equation for predicting biogeochemical pyrite formation in a model CASS ecosystem. In addition to the time-dependent activities of key sulfate-reducing prokaryotic taxa (e.g. Desulfobacteraceae), this equation required methylotrophs (Methylobacteriaceae) and bacterial predators (Bacteriovorax) for best-fitting, suggesting that specific microbial interactions exert meaningful influences on CASS bioremediation efficiency. Our findings confirmed that CASS microorganisms act as an assemblage in response to rewetting by tidewater. Accurate predictions of long-term CASS bioremediation efficiency require modelling of complex and interdependent relationships between geochemical speciation and microbial activity.

Prediction of free metal ion activity in contaminated soils using WHAM VII, baker soil test and solubility model.

Bioavailability and ecotoxicity of metals in contaminated soils depend largely on their solubility. The present investigation was carried out to predict the free ion activity of Zn2+, Cu2+, Ni2+, Pb2+ and Cd2+ in contaminated soils as a function of pH, organic carbon content and extractable metal concentration. Twenty-five composite soil samples were collected from various locations which had a history of receiving sewage sludge (Keshopur and IARI, Delhi), municipal solid waste (Kolkata, West Bengal), polluted river water (Madanpur, Delhi) and industrial effluents (Debari, Rajasthan and Sonepat, Haryana). Four composite soil samples were also collected from adjacent fields which had not received contaminated amendments. Free ion activities (-log10 values), viz. pZn2+, pCu2+, pNi2+, pPb2+ and pCd2+ as measured by the Baker soil test, were 10.1±1.12, 13.4±1.23, 12.9±0.85, 11.6±0.74 and 12.6±2.26, respectively. Free metal ion activities were also determined using the geochemical speciation model WHAM-VII following extraction of soil solution with porous Rhizon samplers from the rhizosphere of growing plants. pH dependent Freundlich model based on soil properties could explain the variation in pZn2+, pCu2+, pNi2+, pPb2+ and pCd2+ to the extent of 84, 52, 73, 60 and 70%, respectively, in the case of data from Rhizon samplers coupled with speciation modelling. Whereas, C-Q model could explain 84, 57, 82, 72 and 74% variability in pZn2+, pCu2+, pNi2+, pPb2+ and pCd2+, respectively, based on soil properties and free metal ion activity as determined with integrated use of Rhizon-WHAM-VII. Modelling approach was superior compared to that based on the Baker soil test solution.

Understanding the leaching behavior of inorganic polymers made of iron rich slags

Abstract This study investigates the leaching of main and trace elements from inorganic polymers made with an iron rich, fumed, > 90% amorphous slag. Different inorganic polymer binders were synthesized, varying the amount of the activating solution and the silica over sodium oxide ratio from 1.6, to 1.8, to 2.0, with constant water content of ±63%. Cascade and column leaching tests were performed in combination with geochemical speciation modeling (Visual MINTEQ) with the aim to understand the speciation of the elements in the inorganic polymer and their leaching behaviour as a function of pH. This would allow to identify the elements which would be the major issue with respect to leaching when the slag will be used in a construction material. The formed inorganic polymer was able to immobilize cationic elements such as barium, copper, magnesium, manganese, and zinc, in the pH range 7–12.5 due to adsorption. Elements such as antimony, arsenic, phosphorus, molybdenum, and vanadium were easily leached out in column and cascade leaching tests, because they most likely occurred as anions in the pore solution. Lead, chromium, and titanium were immobilized in the binder or in crystalline phases in the pH range 3.5–12.5. The study shows that there are multiple factors that affect leaching, the most important of which is shown to be the nature (cationic or anionic) of the elements and the morphology of the matrix. Anions that are present as trace elements (

Geochemistry of thermal waters and arsenic enrichment at Antsirabe, Central Highlands of Madagascar

Thermal waters in the Central Highlands of Madagascar around Antsirabe were investigated using a combination of hydrogeochemical and isotopic methods, geochemical speciation and inverse geochemical modeling. Thermal waters at Antsirabe have temperatures from 35.2 to 47.7 °C, are highly mineralized (EC up to 5.87 mS/cm) are of Na-HCO3-Cl type and have elevated concentrations of arsenic (up to 0.597 mg/l), Sr (up to 8.05 mg/l) and Li (up to 2.83 mg/l). About 25 km west of Antsirabe, a thermal spring at Betafo has a temperature of 53.6 °C, but its mineralization is much lower (EC 0.72 mS/cm) and its water is of Na-HCO3-SO4 type. Concentrations of As, Sr, and Li at Betafo are much lower, only 0.006 mg/l, 0.72 mg/l, and 0.063 mg/l, respectively. Calculated reservoir temperatures using quartz and chalcedony geothermometers are up to 153 °C at Antsirabe and 108 °C at Betafo. Values of δ2H and δ18O are above the WMWL, indicating exchange with silicates like micas. Values of δ13C(DIC) are very enriched (up to −0.45‰) in Antsirabe samples, but depleted with a value of −21.52‰ in a Betafo sample. Values of δ34S(SO4) are close to −4.0‰ in all samples, suggesting an origin of sulfate from a Na-sulfate mineral. Values of 87Sr/86Sr ratios in Antsirabe samples suggest interactions with reservoir rocks. The principal difference between both sites seems to be in the significant input of magmatic-origin CO2 at the Antsirabe site (up to 61.4 mmol/l as determined by inverse geochemical modeling) with resulting higher dissolution rates of As-containing silicate minerals such as micas. There is probably no such input at the Betafo site. In spite of relatively lower As concentrations compared to geothermal waters at global tectonic plate margins, concentrations of As at Antsirabe can represent a serious environmental problem.

Review: mine tailings in an African tropical environment-mechanisms for the bioavailability of heavy metals in soils.

Heavy metals are of environmental significance due to their effect on human health and the ecosystem. One of the major exposure pathways of Heavy metals for humans is through food crops. It is postulated in the literature that when crops are grown in soils which have excessive concentrations of heavy metals, they may absorb elevated levels of these elements thereby endangering consumers. However, due to land scarcity, especially in urban areas of Africa, potentially contaminated land around industrial dumps such as tailings is cultivated with food crops. The lack of regulation for land-usage on or near to mine tailings has not helped this situation. Moreover, most countries in tropical Africa have not defined guideline values for heavy metals in soils for various land uses, and even where such limits exist, they are based on total soil concentrations. However, the risk of uptake of heavy metals by crops or any soil organisms is determined by the bioavailable portion and not the total soil concentration. Therefore, defining bioavailable levels of heavy metals becomes very important in HM risk assessment, but methods used must be specific for particular soil types depending on the dominant sorption phases. Geochemical speciation modelling has proved to be a valuable tool in risk assessment of heavy metal-contaminated soils. Among the notable ones is WHAM (Windermere Humic Aqueous Model). But just like most other geochemical models, it was developed and adapted on temperate soils, and because major controlling variables in soils such as SOM, temperature, redox potential and mineralogy differ between temperate and tropical soils, its predictions on tropical soils may be poor. Validation and adaptation of such models for tropical soils are thus imperative before such they can be used. The latest versions (VI and VII) of WHAM are among the few that consider binding to all major binding phases. WHAM VI and VII are assemblages of three sub-models which describe binding to organic matter, (hydr)oxides of Fe, Al and Mn and clays. They predict free ion concentration, total dissolved ion concentration and organic and inorganic metal ion complexes, in soils, which are all important components for bioavailability and leaching to groundwater ways. Both WHAM VI and VII have been applied in a good number of soils studies with reported promising results. However, all these studies have been on temperate soils and have not been tried on any typical tropical soils. Nonetheless, since WHAM VII considers binding to all major binding phases, including those which are dominant in tropical soils, it would be a valuable tool in risk assessment of heavy metals in tropical soils. A discussion of the contamination of soils with heavy metals, their subsequent bioavailability to crops that are grown in these soils and the methods used to determine various bioavailable phases of heavy metals are presented in this review, with an emphasis on prospective modelling techniques for tropical soils.

GEOCHEMICAL SPECIATION AND BATCH MODE SIMULATION IN THE CARBONATE DEPOSITIONAL ENVIRONMENTS

Geochemical modeling has been frequently used to understand and interpret water-rock interactions in sedimentary basins. Thermodynamic data, kinetic parameters, numerical methods, boundary history, and boundary conditions are factors affecting any geochemical modeling system. In the present study, we have attempted to establish a geochemical speciation model by comparing the interaction of formation water and carbonate rock in the carbonate depositional settings of Cambrian successions of Bachu and Tarim area. A comparative study of geochemical speciation has been performed using four different software: PHREEQCTM, GWBTM, TOUGHREACTTM, and GEODELING. GEODELING is a geochemical code simulator that we have developed, and the details are presented further in this work. All the software has been analyzed minutely, considering the distribution, mobility, and availability of chemical species in geological environments. Very similar results in speciation are observed while working with low-temperature systems. A discrepancy can be observed in the results while working with high temperatures. However, a thorough Newton-Raphson formulation, scaling of algebraic equations and master-species switching helps to reduce the possibility of failures of the numerical method used in PHREEQCTM.