Willem H. van Riemsdijk

Ion binding to natural organic matter : competition, heterogeneity, stoichiometry and thermodynamic consistency

Abstract The general principles of cation binding to humic materials are discussed. Important aspects that need to be included in general purpose speciation models are: the extreme binding heterogeneity of natural humic materials, the variable stoichiometry of binding (monodentate, bidentate and tridentate), the competition between specifically-bound ions, especially protons and metal ions, and electrostatic effects which give rise to ionic strength effects and the non-specific binding of counterions. The NICCA–Donnan model is a semi-empirical model that addresses these issues. It is similar to the previously published NICA–Donnan model except that it introduces an additional degree of scaling that ensures thermodynamic consistency and allows for variable stoichiometry of binding. It implicitly accounts for the large degree of chemical heterogeneity of humic particles. The NICCA (consistent NICA) model also recognizes that the affinity distributions are ion specific and are not fully correlated. The model requires no assumptions to be made about the geometry of the humic particles, but the Donnan submodel does allow for shrinking and swelling. Important model parameters such as the site density and median binding constants ( log K ) are not dependent on pH, metal ion concentration, ionic strength, etc. Data are analysed for H+, Ca2+, Cd2+, Cu2+, Pb2+ and Al3+ binding to a single purified peat humic acid. The NICCA–Donnan model captures the non-linearity of the observed isotherms even at very low free metal ion concentrations. After fitting the model to datasets containing only the proton and one metal ion, the model was able to predict Cd2+–Ca2+, Cu2+–Ca2+ and Pb2+–Al3+ competition reasonably well. It also gave satisfactory predictions of the H+/Mz+ molar exchange ratios. These ratios varied strongly with metal ion: Ca2+ (0.2–0.5); Cd2+ (0.5–1.0); Pb2+ (1.1–1.2); Cu2+ (1.2–1.7) and Al3+ (2.1–2.7), and also to a varying degree with pH and free metal ion concentration.

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.

Metal immobilization in soils using synthetic zeolites

In situ immobilization of heavy metals in contaminated soils is a technique to improve soil quality. Synthetic zeolites are potentially useful additives to bind heavy metals. This study selected the most effective zeolite in cadmium and zinc binding out of six synthetic zeolites (mordenite-type, faujasite-type, zeolite X, zeolite P, and two zeolites A) and one natural zeolite (clinoptilolite). Zeolite A appeared to have the highest binding capacity between pH 5 and 6.5 and was stable above pH 5.5. The second objective of this study was to investigate the effects of zeolite addition on the dissolved organic matter (DOM) concentration. Since zeolites increase soil pH and bind Ca, their application might lead to dispersion of organic matter. In a batch experiment, the DOM concentration increased by a factor of 5 when the pH increased from 6 to 8 as a result of zeolite A addition. A strong increase in DOM was also found in the leachate of soil columns, particularly in the beginning of the experiment. This resulted in higher metal leaching caused by metal-DOM complexes. In contrast, the free ionic concentration of Cd and Zn strongly decreased after the addition of zeolites, which might explain the reduction in metal uptake observed in plant growth experiments. Pretreatment of zeolites with acid (to prevent a pH increase) or Ca (to coagulate organic matter) suppressed the dispersion of organic matter, but also decreased the metal binding capacity of the zeolites due to competition of protons or Ca.

Humic matter and contaminants. General aspects and modeling metal ion binding

Humic substances are soil and fresh-water components that play an important role in the binding and transport of both organic and inorganic contaminants. Transport of the contaminants due to ground- and fresh-water dynamics is directly related to the risks associated with contaminations. The mobility of soluble humic substances is related to their interaction with soil mineral particles. Some key references for the binding of organic and inorganic contaminants and for the binding of humics to mineral particles are presented. Humic substances also play a role in the analysis of the contaminants in natural waters and with remediation of water or soil polluted with pesticides, heavy metal ions, and radionuclides. These aspects are illustrated with some examples. The problems that are encountered with the modeling of the binding of contaminants to humics and of heavy metal ions in particular are illustrated by considering the nonideal competitive adsorption model (NICA) extended with electrostatic interactions. The NICA-Donnan model gives quite good results for the description of metal ion binding, as is illustrated for metal ion binding to purified peat humic acid (PPHA). Finally, some remarks are made with respect to the use of the NICA-Donnan model in general purpose speciation programs and of simplified versions of the model for predictions under restricted environmental conditions.

Phytotoxicity and bioavailability of nickel: Chemical speciation and bioaccumulation

The effect of pH on the bioaccumulation of nickel (Ni) by plants is opposite when using a nutrient solution or a soil as a growing medium. This paradox can be understood if the pH effect on the bioaccumulation, on the chemical speciation in the soil solution, and on the binding to the soil of Ni are all taken into account. Using simple equations to describe the individual relationships, it is possible to quantify these effects once the relationships have been established. Increased Ni uptake leads to reduced plant dry weight production for a certain growing period. The median effective concentration (EC50) decreased from 23 to 1.7 microM Ni in the nutrient solution for pH 4.0 to 7.0, whereas the EC50 of added Ni in a sandy soil increased from 0.72 to 9.95 mmol Ni/kg soil for pH 4.7 to 6.8. Bioaccumulation, binding to the soil solid phase, and binding to the dissolved organic matter all increase with increasing pH. However, the magnitude of the effect is the least for bioaccumulation as a function of pH, causing the apparent paradox.

Application of 31P and 27Al MAS NMR for phosphate speciation studies in soil and aluminium hydroxides: promises and constraints

Magic angle spinning (MAS) nuclear magnetic resonance (NMR) was used to investigate the chemical environment of P in soil and soil components. 31P and 27Al MAS NMR spectra are presented of synthetic aluminium hydroxides (amorphous aluminium hydroxide and gibbsite), reacted with P under different conditions of P concentration, temperature and pH. The reaction product is amorphous octahedral aluminium phosphate, which transforms (partly reversibly) to tetrahedral aluminium phosphate upon drying. Results of several experiments on excessively fertilized sandy soil material are discussed. The soil particle fraction smaller than 50 μm was used for NMR analysis. 31P and 27Al MAS NMR spectra show a CaP pool and an AlP pool. A six-fold water extraction removes part of both P pools. Oxalate extraction removes all CaP and AlP from the sample. Removal of the labile P pool by the HFO-DMT long-term P desorption technique, does not drastically change the 31P MAS NMR spectrum. The formerly mentioned CaP and AlP are thus stable P pools. The 1H31P cross-polarization (CP) spectrum of the original soil sample revealed a third chemical environment, which was identified as labile CaP: this P pool does not appear in the 1H31P CP spectrum of the soil sample from which the fast P pool had been removed. The combination of MAS and CP MAS NMR can thus reveal at least three different P species in soil, of which two pools were identified as stable, and one as labile.

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.

Effects of humic acid and competing cations on metal uptake by Lolium perenne

Within the biotic ligand model, which describes relationships between chemical speciation and metal binding at an organisms surface, multicomponent (long-term) metal uptake by plants has seldom been studied. In the present work, we exposed perennial ryegrass to nutrient solutions with two levels of Cd, Cu, Ni, Pb, and Zn (1 and 0.1 microM) and with or without 30 mg/L of humic acid. Iron and Mn concentrations were constant over all treatments. The hypothesis tested was that humic acid lowers the free and labile metal concentration and, therefore, reduces the metal uptake and, finally, the metal content of the plant. The free metal ion concentrations in the nutrient solutions were measured by the Donnan membrane technique and labile metal concentrations by diffusive gradients in thin-films. The metal content of the shoots depends on the metal content of the roots. The metal content of the roots is a function of the adsorption of metals on the root surface. In a multicomponent system at metal concentrations of 1 microM, humic acid decreased Cu, Pb, and Fe adsorption at the root surface, but it increased Cd, Zn, and Mn adsorption at the root surface. Complexation of cations such as Cu, Pb, and Fe with high affinity for (dissolved) organic matter may lead to increased uptake of cations with low affinity for organic matter (Ni, Zn, and Cd) because of competition between cations at the root surface. The results suggest that competition between metal ions can play a major role in multicomponent metal uptake, which has to be taken into account during risk assessments of metal-polluted soils.

Metal uptake by Lolium perenne in contaminated soils using a four-step approach.

Most research dealing with soil (solution) speciation and metal uptake by plants has focused on the relationships between a certain bioavailable fraction in the soil and metal uptake by aboveground parts of the plants. Here, a new approach to interpretation of metal uptake is presented that considers four steps: First, the metal concentration in the soil solution is related to the total metal content of the soil. Second, the metal adsorption to the root surface is related to the metal concentration in the soil solution. Third, the metal content in the roots is related to the adsorption of metal ions to the root surface. Fourth, the metal content in the shoots is related to the metal content in the roots. For grass grown on 10 different soils, it is shown that the metal adsorption to the root surface is pH-dependently related to the free or total metal concentration in the soil solution. The metal content in the roots depends linearly on the metal adsorption at the root surface, whereas the metal content in the shoots depends on the metal content in the roots, either linearly (Zn) or reaching a maximum (Cu, Pb, and Cd). For the Ni content in the shoots as a function of the root content, the relation is pH dependent, probably because of the competition effects of Ca. The pH of the soil has to be taken into account when CaCl2 extractions are used as a basis for risk assessment toward plants.

Relationship between Metal Speciation in Soil Solution and Metal Adsorption at the Root Surface of Ryegrass

The total metal content of the soil or total metal concentration in the soil solution is not always a good indicator for metal availability to plants. Therefore, several speciation techniques have been developed that measure a defined fraction of the total metal concentration in the soil solution. In this study the Donnan Membrane Technique (DMT) was used to measure free metal ion concentrations in CaCl(2) extractions (to mimic the soil solution, and to work under standardized conditions) of 10 different soils, whereas diffusive gradients in thin-films (DGT) and scanning chronopotentiometry (SCP) were used to measure the sum of free and labile metal concentrations in the CaCl(2) extracts. The DGT device was also exposed directly to the (wetted) soil (soil-DGT). The metal concentrations measured with the speciation techniques are related to the metal adsorption at the root surface of ryegrass (Lolium perenne L.), to be able to subsequently predict metal uptake. In most cases the metal adsorption related pH-dependently to the metal concentrations measured by DMT, SCP, and DGT in the CaCl(2) extract. However, the relationship between metal adsorption at the root surface and the metal concentrations measured by the soil-DGT was not-or only slightly-pH dependent. The correlations between metal adsorption at the root surface and metal speciation detected by different speciation techniques allow discussion about rate limiting steps in biouptake and the contribution of metal complexes to metal bioavailability.