Hans Sticher

Experimental determination of colloid deposition rates and collision efficiencies in natural porous media

Mobile colloids in groundwater aquifers and soils can serve as carriers for strongly sorbing contaminants and thereby facilitate contaminant transport. Therefore mobile colloids may have to be considered in modeling the fate of strongly sorbing contaminants in subsurface environments. In this study we present a Chromatographic short-pulse technique for measuring colloid deposition rate coefficients and experimental collision efficiencies in natural porous media. The method was evaluated using four different experimental systems of increasing complexity. Short pulses (equivalent to 0.002 to 0.03 pore volumes) of latex or humic-coated hematite suspensions were injected under saturated flow conditions into laboratory columns packed with glass beads, soil, or aquifer materials. Colloid breakthrough curves were measured on-line using fluorescence and UVVIS spectrophotometers. Deposition rate coefficients determined with the short-pulse method were in excellent agreement with results from step-input experiments. Experiments with different flow rates and column dimensions showed that colloid deposition generally followed a first-order kinetic rate law. On the basis of experimental fast deposition rates, collision efficiencies for colloid deposition can be calculated. The results demonstrate that the short-pulse method can be used very efficiently for studying the effects of solution chemistry and flow velocity on the kinetics of colloid deposition in natural porous media. The short-pulse method has several advantages over the more traditionally used step-input experiment and allows running several experiments on a single column without significant blocking or filter ripening effects.

Release of phenols from Lupinus albus L. roots exposed to Cu and their possible role in Cu detoxification

The mechanisms enabling plants to tolerate high concentrations of available Cu in their rhizosphere are still poorly understood. To better understand the mechanisms involved, Lupinus albus L. (white lupin) was grown over 40 days in a hydroponic system compelling roots to develop under sterile conditions in the presence of a nutrient solution containing 0.5, 20 or 62 μM Cu. The following parameters were investigated in detail: low molecular weight phenols in nutrient solution (colorimetric assay), high molecular weight phenols in roots and in solution (HPLC-MS, HPLC-UV), pH, redox potential in solution (electrochemistry) and Cu distribution in the plant (AAS) as well as in apical root sections (EDX microanalysis). Finally, in vitro adsorption studies using voltammetry were conducted to evaluate the Cu adsorption behaviour of different phenolic compounds. When exposed to 62 μM Cu, biomass production of white lupin was strongly reduced. Plants grown in the presence of 20 μM Cu had a similar dry matter production compared to the control plants grown in a 0.5 μM Cu solution. However, an increased release of soluble and high molecular weight phenols into the solution was observed. The concentration of polyphenolic compounds in the roots (particularly isoflavonoids like genistein and genistein-(malonyl)-glucoside) was significantly higher for lupins grown in a 20 μM Cu solution compared to the control plants. As shown by an in vitro adsorption study, these phenolic compounds can bind Cu ions. In addition, plants exposed to 20 and 62 μM Cu cumulated high Cu amounts in root cell walls whereas only low amounts reached the symplasm. Therefore, it is proposed that the complexation of Cu2+ ions in the rhizosphere and in the roots apoplasm by phenolic compounds could alleviate Cu-mediated toxicity.

LIGAND EFFECT ON THE ADSORPTION OF HEAVY METALS : THE SULFATE - CADMIUM - GOETHITE CASE

AbsractComplexing ligands may alter the adsorption behaviour of heavy metals compared to a ligand free system, either by changes in solution speciation, or by interactions at the adsorbing mineral surface. In this paper the effect of sulfate on the adsorption of cadmium onto goethite has been studied. Adsorption experiments under defined conditions revealed the following effects: Cadmium adsorption from sulfate containing solutions by goethite was higher than from pure NaNO3-solutions. Simultaneously sulfate adsorption increased, indicating synergic effects. Increasing the ionic strength of the metal-sulfate solutions caused a decrease both in heavy metal and in sulfate adsorption.Model calculations on simultaneous adsorption of cadmium and sulfate lead to the following conclusions: 1.) Sulfate adsorption reduces the surface potential so that the surface becomes more attractive to heavy metals. 2.) In addition, new, so called ternary surface complexes (SOCd+-SO42−) between reactive surface sites, heavy metals and sulfate-anions must be assumed in order to get agreement between observed and calculated effects.

Colloid transport in natural porous media: influence of surface chemistry and flow velocity

Abstract Mobile colloids in soils and groundwater aquifers can act as carriers for strongly sorbing chemicals and may enhance contaminant transport. Field studies suggest that mobile colloids can include clay minerals, oxides or hydroxides of Fe and Al, colloidal silica, organic matter, and biocolloids such as viruses and bacteria. The transport of colloids through porous media strongly depends on the kinetics of colloid deposition and release. Important chemical factors controlling colloid deposition and release kinetics in natural porous media are the charge of matrix and colloid surfaces and electrolyte type and concentration in the solution phase. The surface charge is strongly influenced by solution pH and the presence of specifically adsorbing ions, including organic polyelectrolytes such as humic substances. Surface charge heterogeneities on colloids and matrix surfaces also play an important role. Colloid transport experiments conducted under well-controlled conditions were conducted to investigate the influence of surface chemistry, solution chemistry, and flow velocity on the kinetics of colloid deposition in natural porous media.

Measurement of Sorption Isotherms with Flow-Through Reactors

A simple flow-through reactor technique is described for measurements of sorption isotherms on solid environmental materials such as soils, crushed rocks, aquifer materials, or peat. The applicability of the method was established by determining copper sorption isotherms at effectively constant pH for two different soil samples. The experimental results compare favorably with independent batch and column studies. Potential advantages of the flow-through technique are exemplified by studying the sorption behavior of one soil sample that shows in batch experiments a pronounced dependence on the solid concentration (particle concentration effect). Comparison of these results with the flow-through reactor technique and independent column experiments confirms unequivocally that in this case the influence of the solid concentration is an artifact due to insufficient pre-washing of the soil with the background electrolyte solution and has nothing to do with the shaking process in batch experiments. The present results indicate that incomplete removal of preadsorbed ions may often represent a major source of error in sorption studies and may be difficult to recognize in batch experiments.

Convective transport of acids and bases in porous media

We present a convective description of acid-base transport in porous media which is based on classical one-component nonlinear Chromatographic theory applied to the acidity of the system. In the mobile phase the solution acidity is given by c = [H+]t - [OH−]t, where [H+]t and [OH−]t are the total solution concentrations of H+ and OH−, respectively. In the stationary phase the surface acidity corresponds to the charge density σ, which is commonly presented as a function of pH of the solution. The response of a Chromatographic column to a step pH change at column inlet results in a pH breakthrough curve which consists of a combined Chromatographic front. This combined front begins with a diffuse subfront and ends with a self-sharpening subfront. The present Chromatographic theory is used to predict experimentally observed pH breakthrough curves for columns filled with materials of known pH-dependent charging behavior. Good agreement between theoretical predictions and experimental results is observed. Essentially, the same Chromatographic theory is used to explain pH breakthrough curves when the salinity of the input solution is changed. This situation leads usually to an additional nonretarded front where a change in pH occurs.

Cation transport in natural porous media on laboratory scale: multicomponent effects

Abstract Multicomponent transport experiments were performed with four major cations, Na + , K + , Ca 2+ and Mg 2+ , in laboratory columns packed with a non-calcereous soil. The breakthrough curves are explained quantitatively with a box model including cation exchange. We use a single set of selectivity coefficients, an independently verified value of the cation-exchange capacity (CEC), and an adjusted value of the Peclet number. This Peclet number is smaller than the value determined from independent tracer experiments. The model is able to predict all experimentally observed breakthrough curves quite well. The selectivity coefficients determined from binary exchange experiments prove unreliable for the prediction of multicomponent experiments. We propose to estimate the selectivity coefficients by directly fitting the multicomponent breakthrough curves. Their shape is a very sensitive function of the values of these coefficients. Concepts from non-linear chromatography can be used in order to interpret several qualitative features of the breakthrough curves.

Quantitative description of multi-component reactive transport in porous media: An empirical approach

We propose a general method for the description of multi-component reactive transport in porous media. The approach is based on the local equilibrium assumption and consists of four steps: (i) measurement of the adsorbed amounts of all components of interest by means of laboratory batch experiments, (ii) construction of a simple, empirical binding model consistent with the experimental data by means of constrained, regularized least-squares, (iii) calculations of multi-component breakthrough curves based on numerical solutions of the transport equations coupled to the empirical binding model, (iv) verification of these model predictions with laboratory column experiments. We demonstrate the applicability of the procedure by investigating simultaneous transport of cadmium and zinc at constant pH in a silica sand column.

Untersuchung der Wurzelexsudation nach unterschiedlicher Eisen- und Kupferversorgung am Beispiel von Lupinus albus L.

Providing Lupinus albus L. with nutrient solutions varying in their iron and copper content resulted in different root exudation patterns. Iron deficiency caused a high release of phenolic compounds and protons and an accumulation of copper in the roots. After plant exposition to copper deficiency, increased amounts of dissolved organic carbon in the medium were measured. Plants grown under excess of copper accumulated the metal in the roots and translocated a part to the shoot. Additionally, higher amounts of phenolics in the medium were found.