Silicic acid competes for dimethylarsinic acid (DMA) immobilization by the iron hydroxide plaque mineral goethite.

A surface complexation modeling approach was used to extend the knowledge about processes that affect the availability of dimethylarsinic acid (DMA) in the soil rhizosphere in presence of a strong sorbent, e.g., Fe plaques on rice roots. Published spectroscopic and molecular modeling information suggest for the organoarsenical agent to form bidentate-binuclear inner-sphere surface complexes with Fe hydroxides similar to the inorganic As oxyanions. However, since also the ubiquitous silicic acid oxyanion form the same bidentate binuclear surface complexes, our hypothesis was that it may have an effect on the adsorption of DMA by Fe hydroxides in soil. Our experimental batch equilibrium data show that DMA is strongly adsorbed in the acidic pH range, with a steep adsorption edge in the circumneutral pH region between the DMA acidity constant (pKa=6.3) and the point of zero charge value of the goethite adsorbent (pHpzc=8.6). A 1-pK CD-MUSIC surface complexation model was chosen to fit the experimental adsorption vs. pH data. The same was done for silicic acid batch equilibrium data with our goethite adsorbent. Both model parameters for individual DMA and silicic acid adsorption were then merged into one CD-MUSIC model to predict the binary DMA+Si adsorption behavior. Silicic acid (500 µM) was thus predicted by the model to strongly compete for DMA with up to 60% mobilization of the latter at a pH6. This model result could be verified subsequently by experimental batch equilibrium data with zero adjustable parameters. The thus quantified antagonistic relation between DMA and silicic acid is discussed as one of factors to explain the increase of the DMA proportion in rice grains as observed upon silica fertilization of rice fields.

Investigation on stability and preservation of antimonite in iron rich water samples.

The stability of antimonite in iron rich water samples is rather poor. The aim of the study was to find a simple procedure by using preservation agents to keep the speciation information from sampling till analysis. Species analysis of antimony traces (lower µg L(-1) range) was done by HPLC-ICP-MS. Phosphoric acid, tartrate, and EDTA were tested as preservation agents in comparison to no addition. The use of EDTA as the preservation agent provided the best results. The suggested procedure is to add 20 mM EDTA as final concentration immediately during sampling and store them at dark and cool (6 °C) as usual. Using this procedure, the stability of Sb(III) as well as of Sb(V) was proven for at least 7 days, even for high iron concentrations.

Remobilization of pentavalent antimony and vanadium from a granular iron hydroxide material--a comparative study of different leaching systems.

The remobilization of antimony and vanadium from previously loaded commercial granular ferric-hydroxide GEH material (intended for water treatment) was examined by using a sequential extraction procedure and three different leaching systems to evaluate their physicochemical mobility and potential availability under different simulated environmental conditions. A classical batch extraction, an extraction cell (EC) and rotating-coiled columns (RCC) were used as extraction systems. For each system it could be shown that the content of ion-exchangeable antimony and vanadium in previously loaded material is negligible (<1.5%). The oxyanions were sorbed strongly and could be predominantly remobilized through reducing agents, which means through dissolution of the iron (hydr)oxide matrix. The major advantages of dynamic systems in comparison to batchwise fractionation technique are the drastically reduced extraction time and the possibility of generating information to the leaching kinetics. It is shown that the efficiency of the three leaching systems is quite different employing Wenzel's sequential fractionation protocol. Only by working with RCC, the iron (hydr)oxide matrix was completely dissolved within four steps resulting in the total mobilization of antimony and vanadium. EC seems to be less suitable for leaching studies of Sb and V sorbed on iron(hydr)oxide. The remobilizable proportion of the several fractions was lower in comparison to batch and RCC and seems to be a result of an agglomeration of the GEH in the EC device.

Biogenic Fe(III) minerals lower the efficiency of iron-mineral-based commercial filter systems for arsenic removal.

Millions of people worldwide are affected by As (arsenic) contaminated groundwater. Fe(III) (oxy)hydroxides sorb As efficiently and are therefore used in water purification filters. Commercial filters containing abiogenic Fe(III) (oxy)hydroxides (GEH) showed varying As removal, and it was unclear whether Fe(II)-oxidizing bacteria influenced filter efficiency. We found up to 10(7) Fe(II)-oxidizing bacteria/g dry-weight in GEH-filters and determined the performance of filter material in the presence and absence of Fe(II)-oxidizing bacteria. GEH-material sorbed 1.7 mmol As(V)/g Fe and was ~8 times more efficient than biogenic Fe(III) minerals that sorbed only 208.3 µmol As(V)/g Fe. This was also ~5 times more efficient than a 10:1-mixture of GEH-material and biogenic Fe(III) minerals that bound 322.6 µmol As(V)/g Fe. Coprecipitation of As(V) with biogenic Fe(III) minerals removed 343.0 µmol As(V)/g Fe, while As removal by coprecipitation with biogenic minerals in the presence of GEH-material was slightly less efficient as GEH-material only and yielded 1.5 mmol As(V)/g Fe. The present study thus suggests that the formation of biogenic Fe(III) minerals lowers rather than increases As removal efficiency of the filters probably due to the repulsion of the negatively charged arsenate by the negatively charged biogenic minerals. For this reason we recommend excluding microorganisms from filters (e.g., by activated carbon filters) to maintain their high As removal capacity.

Investigation on stability and preservation of arsenic species in iron rich water samples.

Important for the accurate and reproducible determination of inorganic redox forms of arsenic in iron-rich waters is their conservation prior to analysis. Species and trace element analysis methods are commonly laboratory based. Stabilisation of samples is necessary for subsequent laboratory analysis in order to preserve the information about the system from which the samples were taken. Pre-treatment procedures based on complexation of metal ions and moderate acidification of the samples are presented. The concentration of the stabilisation agent and the storage temperature were optimised. The most satisfactory results were obtained with 0.01 mol l(-1) phosphoric acid and a storage at 6 degrees C.