J.C.L. Meeussen

Adsorption of fulvic acid on goethite

The adsorption of fulvic acid by goethite was determined experimentally as a function of concentration, pH, and ionic strength. The data were described with the CD-MUSIC model of Hiemstra and Van Riemsdijk (1996), which allows the distribution of charge of the bound fulvate molecule over a surface region. Simultaneously, the concentration, pH, and salt dependency of the binding of fulvic acid can be described. Using the same parameters, the basic charging behavior of the goethite in the absence of fulvic acid could be described well. The surface species used in the model indicate that inner sphere coordination of carboxylic groups of the fulvate molecule is important at low pH, whereas at high pH the outer sphere coordination with reactive groups of the fulvate molecule with high proton affinity is important.

Identification and geochemical modeling of processes controlling leaching of Cr(VI) and other major elements from chromite ore processing residue

Abstract Chromite ore processing residue (COPR) contains very high levels of chromium as Cr(III) and Cr(VI) and has a pH of ∼11.5 to 12. Millions of tonnes of COPR have in the past been deposited in urban areas. We have studied the factors that control leaching of Cr(VI), Ca, Al, Si, and Mg from COPR by means of batch experiments, mineralogical characterization of COPR via X-ray powder diffraction and scanning electron microscopy, and chemical equilibrium modeling. Batch experiments at a range of pH values and two liquid:solid ratios showed that mineral solubility control exists for aqueous concentrations of Cr(VI) above pH 10. Calculations indicate that the solid phases that control the solubility of Cr(VI) at pH values above 11 are Cr(VI)-substituted hydrogarnet (Ca3Al2(H4O4,CrO4)3) and Cr(VI)-hydrocalumite (Ca4Al2(OH)12CrO4·6 H2O), a layered double-hydroxide clay with chromate anions held in the interlayers. In the pH range 9.5 to 11, the description of the Cr(VI) concentration in solution was strongly improved by the incorporation in the model of Cr(VI)-ettringite (Ca6Al2(OH)12(CrO4)3·26 H2O), which precipitates as a secondary phase when hydrocalumite dissolves. The proposed model for leaching of COPR at high pH includes Cr(VI)-bearing hydrogarnet, Cr(VI)-hydrocalumite, Cr(VI)-ettringite, brucite, calcite, Ca2Al2(OH)10·3 H2O, CaH2SiO4, and gehlenite hydrate (Ca2Al2(OH)6SiO8H8·H2O). The model accurately predicts the concentrations of Cr(VI), Ca, Al, Si, and Mg in solution in the pH range 10 to 12 as well as the pH-buffering behavior. Below pH 8, a decrease in the Cr(VI) concentration in solution is observed, which may be attributed to sorption of chromate onto freshly precipitated Al and Fe hydroxide surfaces. Sulfate and carbonate show the same type of behavior as chromate. The chemistry of COPR shows similarities with cement and high-pH municipal waste incinerator bottom ash.

Biodegradation of metal cyanides by mixed and pure cultures of fungi

Abstract Former gasworks sites are sometimes be heavily contaminated with spent oxide which contains cyanide complexed to metals (especially iron). In this study, mixed fungal cultures have been isolated from acidic gasworks soil by their ability to utilize iron or nickel cyanide as the sole source of nitrogen at acidic or neutral pH, respectively. A mixed culture comprising Fusarium solani and Trichoderma polysporum was obtained by enrichment on tetracyanonickelate [K 2 Ni(CN) 4 ] at pH 4. A second mixed culture consisting of Fusarium oxysporum, Scytalidium thermophilum , and Penicillium miczynski was isolated on hexacyanoferrate [K 4 Fe(CN) 6 ] also at pH 4. Both consortia were able to grow on K 4 Fe(CN) 6 as the sole source of nitrogen under acidic conditions. Growth was associated with progressive removal of cyanide from the culture supernatant. After the termination of growth, at least 50% of the total cyanide had been degraded. Growth of the fungi on K 2 Ni( 14 CN) 4 as a source of nitrogen at pH 7 yielded 14 C-labelled carbon dioxide. Growth of the Fusarium isolates on K 2 Ni(CN) 4 at pH 7, associated with the removal of cyanide, required 5 days as compared to 28 days on K 4 Fe(CN) 6 at pH 4. Cyanide uptake by the fungi on K 4 Fe(CN) 6 at pH 4 occurred simultaneously with removal of iron from the biomass-free medium. Pure cultures of F. solani and F. oxysporum were grown on K 2 Ni(CN) 4 or K 4 Fe(CN) 6 in pure culture at pH 7 or 4, respectively.

Use of solid phase characterisation and chemical modelling for assessing the behaviour of arsenic in contaminated soils

A soil, containing waste material from an industrially contaminated site, was found to be heavily contaminated with several heavy metals and As. A risk assessment for As leaching from this material has been carried out in several stages, collation and examination of historical records, solid-phase characterization and chemical modelling. The historical record indicates that the most probable source of As was arsenopyrite. However, the solid phase characterization of the soil, using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive microanalysis (EDAX), did not yield any direct evidence for pyritic phases, although there was clear evidence of known pyrite-weathering products, such as jarosite. The relative stability of pyrite and arsenopyrite have been modelled for the range of acidity and redox potentials likely to be encountered on the site. For adsorption modelling, a surface complexation model was used to predict arsenate desorption as a function of pH. It was assumed that the principal reactive adsorbent for As was hydrous ferric oxide (HFO) and this assumption was supported by the results of direct and indirect measurements and by the mineral stability calculations. This approach was successful at predicting the increased mobility of As at increasingly alkaline conditions. The modelling predictions were supported by results from batch equilibration experiments. Thus, it was possible to link direct observations of mineralogy, mineral stability calculations and adsorption models in order to predict the mobility of As. The success of this approach was dependent on identifying the reactive phase in this particular soil and having the appropriate data required for the adsorption modelling.

Modeling the binding of fulvic acid by goethite: the speciation of adsorbed FA molecules

Abstract Under natural conditions, the adsorption of ions at the solid–water interface may be strongly influenced by the adsorption of organic matter. In this paper, we describe the adsorption of fulvic acid (FA) by metal(hydr)oxide surfaces with a heterogeneous surface complexation model, the ligand and charge distribution (LCD) model. The model is a self-consistent combination of the nonideal competitive adsorption (NICA) equation and the CD-MUSIC model. The LCD model can describe simultaneously the concentration, pH, and salt dependency of the adsorption with a minimum of only three adjustable parameters. Furthermore, the model predicts the coadsorption of protons accurately for an extended range of conditions. Surface speciation calculations show that almost all hydroxyl groups of the adsorbed FA molecules are involved in outer sphere complexation reactions. The carboxylic groups of the adsorbed FA molecule form inner and outer sphere complexes. Furthermore, part of the carboxylate groups remain noncoordinated and deprotonated.

The effect of EDTA and fulvic acid on Cd, Zn, and Cu toxicity to a bioluminescent construct (pUCD607) of Escherichia coli

The hypothesis, that metal toxicity is dominated by free ion activity, was tested by comparing calculated metal activities with measured toxic responses to a genetically modified, luminescent bacterium, Escherichia coli. The toxicity of Cd, Cu, and Zn sulphate salts in the presence of EDTA and fulvic acid in well-defined solutions was measured. Good agreement between free metal activity and toxicity was found for Cu but not for Zn and Cd. The toxicity relationships were altered by glucose addition to the organism. Stable chloride complexes may have contributed to the toxicity of Cd under the test conditions. The results suggest that there is not always a simple relationship between toxicity and free-ion metal concentration and that further account should be taken of competitive interactions between living cells and ligands and the physiological status of the organism.

Transport of complexed cyanide in soil

Abstract Contamination of soil with cyanide, generally in the form of iron cyanide complexes [Fe(CN) 6 3− and Fe(CN) 6 4− ], is commonly found at several types of industrial sites. The risks for human health or the environment posed by such sites are largely determined by the chemical behaviour and transport of complexed cyanide in soil. In acidic soil this behaviour is probably dominated by equilibrium with Prussian blue, Fe 4 (Fe(CN) 6 ) 3 (s), which is sparingly soluble under acidic conditions and limits dissolved cyanide concentrations and mobility. However, at pH levels higher than about 7 the solubility of this precipitate is extremely high, which allows cyanide to be very mobile under such conditions. Nevertheless, according to field observations, Prussian blue appears to persist for decades in alkaline soils. In this paper multicomponent transport calculations, including equilibrium with Prussian blue, iron hydroxide and relevant redox reactions, are compared with the situation as observed in two contaminated soil profiles. In case of an acidic soil, there appears to be good agreement between the calculations and the field situation. Here the mobility of cyanide is mainly regulated by the pH and redox potential and is relatively low. In case of an alkaline soil, precipitation of Prussian blue will not take place and cyanide will be very mobile once leached from the initial waste material. In this case the dissolution rate of Prussian blue is the process which determines cyanide concentrations in the groundwater. The calculations show that this dissolution rate is determined by the acid neutralizing capacity of the solution infiltrating in the waste material. The buffer capacity depends on the soil composition and can easily be so low that complete dissolution of Prussian blue may take decades. This very likely explains the persistence of this material in alkaline soils.

TRANSPORT OF MALONATE IN A GOETHITE-SILICA SAND SYSTEM

Abstract The mobility of weak organic acids is of great importance for the transport of heavy metals and other contaminants. This work discusses the breakthrough behavior of malonic acid through a column packed with goethite coated sand. The transport of malonic acid is dominated by multicomponent chemical interactions of protons and the malonic acid on the goethite surface and in solution. The experimental results show a fast breakthrough and a slow desorption of malonate. The binding capacity of the goethite coated silica sand column for malonic acid and acidity are discussed and their impact on elution is illustrated. The data have been predicted with a combination of the CD-MUSIC model and a one-dimensional convective-dispersive transport code. Reasonable agreement was obtained between data and model predictions.

Transport of ions in physically heterogeneous systems; convection and diffusion in a column filled with alginate gel beads, predicted by a two-region model

Abstract An experimental flow system was developed to simulate solute transport in heterogeneous porous media, in which physical nonequilibrium occurs. The system consists of a column filled with spherical alginate gel beads. The gel acts as a stagnant water phase, in which ions can only move by diffusion. First, the release of nitrate and phosphate ions from the gel beads by diffusion was measured separately, in a batch system. Model calculations showed that it is possible to accurately predict the release of ions from the gel beads, by applying Fick’s law for diffusion and reported diffusion coefficients of the ions in water. Subsequently, the transport of ions in the column was tested against model calculations. The breakthrough of nitrate from a bead filled column was measured at different flow rates. The breakthrough curves were successfully predicted with a two-region model. In this model the solution in the beads is represented by an immobile region and the solution in the pores by a mobile region. Mass transport takes place by convection in the mobile region, and by diffusion in the immobile region. The model only requires the following independent input parameters: the flow rate, the diffusion coefficient in water, the column dimensions and the bead radius.

Mobilization of Heavy Metals in Soils By Oxidation of Sulphides

Heavy metals like zinc are encapsulated in ironsulphide minerals (Pyrite). Nitrate addition to soils may change the environmental conditions so that the ironsulphides can be oxydized, which increases the mobility of zinc. The oxidation process in a soil was described with a multi-component transport model, and results (high zinc concentrations in the subsoil) correspond qualitatively to field measurements.