Hexafluorosilicate anion in the formation of a coordination cage: anion competition

The construction of a coordination cage in a system of dual anions including hexafluorosilicate was investigated. Significant catalytic effects on catechol oxidation catalysis performance ranks among the most efficient yet recorded.

Solubility and transport of phosphate and the accompanying ions as influenced by sulphate salts in a model calcareous soil system

Plant availability of phosphorus (P) in calcareous soil can be improved by modifying the chemical environment of the soil-P fertilizer reaction zone through the banding of non-phosphate fertilizer with P. We investigated the solubility and diffusive transport of P as influenced by addition of two sulphate salts [(NH4)2SO4 and K2SO4] to NH4H2PO4 and KH2PO4. The salts were applied to a series of wax columns packed with approximately 223 g of Ca2+-saturated cation exchange resin-sand mixture buffered with CaCO3. The background pH of the mixture was 8.8. Each treatment contained approximately 204.5 mg P kg-1 soil, while 632.3 mg SO42- kg-1 soil was added to each P source for treatments containing the dual bands to provide a molar concentration of P and SO42- of 6.6 mmol kg-1 soil. After 2 wk of incubation, column segmentation and extraction showed that H+ moved deeper into the columns on addition of the sulphate salts compared with adding NH4H2PO4 or KH2PO4 alone. The maximum depth of P penetration in the columns containing NH4H2PO4 was 4.2 cm, while P transport in the columns treated with KH2PO4 was restricted to the top 4.0 cm depth. Addition of (NH4)2SO4 or K2SO4 to NH4H2PO4 increased the concentration of water-extractable P by 43 and 21%, respectively, above that in NH4H2PO4 alone. Similarly, addition of (NH4)2SO4 or K2SO4 increased the concentration of water-soluble P by 48 and 41%, respectively, above the amount in KH2PO4 alone. The increased water solubility of P on addition of the sulphate salts was attributed to anion competition between HPO42- and SO42- for precipitation with Ca2+. We also observed ionic competition between NH4+ and K+ when both cations were added together, causing K+ to travel farther into the column and with increased solubility than when applied alone. Our results showed that anion and cation competition can be used to modify the transport and solution concentration of ions through dual banding. These results also suggested that the combination of anion competition by SO42- and pH reduction due to salt effect could have a positive influence on the availability of P in calcareous soils. Key words: Phosphate, sulphate, columns, solubility, diffusive transport, resin

Bicarbonate-dependent and bicarbonate-independent mechanisms contribute to nondiffusive uptake of acetate in the ruminal epithelium of sheep

The present study investigated the significance of apical transport proteins for ruminal acetate absorption and their interaction with different anions. In anion competition experiments in the washed reticulorumen, chloride disappearance rate (initial concentration, 28 mM) was inhibited by the presence of a short-chain fatty acid mixture (15 or 30 mM of each acetate, propionate, and butyrate). Disappearance rates of acetate and propionate, but not butyrate (initial concentration, 25 mM each) were diminished by 40 or 80 mM chloride. In isolated ovine ruminal epithelia mounted in Ussing chambers, an increase in chloride concentration from 4.5 to 90 mM led to a decrease of apical acetate uptake at a concentration of 0.5 mM. Mucosal nitrate inhibited acetate uptake most potently whereas sulfate had no effect. Decreasing mucosal pH from 7.4 to 6.1 approximately doubled uptake of acetate both at 0.5 and 10 mM, but this doubling was almost abolished when HCO3− was absent. The stimulated uptake at mucosal pH 6.1 consisted of a bicarbonate-dependent, nitrate-inhibitable part ( Km = 54 mM) and a bicarbonate-independent component ( Km = 12 mM) that was also sensitive to nitrate inhibition. Maximal uptake was three times larger for bicarbonate-dependent vs. bicarbonate-independent uptake. Mucosal addition of 200 μM DIDS, 400 μM p-chloromercuribenzene sulfonic acid, 800 μM p-hydroxymercuribenzoic acid, or 100 μM phloretin had no effects on acetate uptake although the latter two inhibited l-lactate uptake. Our data conclusively show a dominant involvement of proteins in apical acetate uptake. Previously described pH effects on acetate absorption originate mainly from modulation of acetate/bicarbonate exchange. Additionally, there is bicarbonate-independent uptake of acetate anions that is protein coupled but not via monocarboxylate cotransporter.

The effect of ionic strength variation and anion competition on the development of nitrate accumulations in variable charge subsoils

The leaching of N fertilisers has led to the formation of nitrate (NO3) accumulations in deep subsoils (>5 m depth) of the Johnstone River catchment. This paper outlines the chemical mechanism by which these NO3 accumulations are formed and maintained. This was achieved via a series of column experiments designed to investigate NO3 leaching in relation to the soil charge chemistry and the competition of anions for exchange sites. The presence of variable charge minerals has led to the formation positive surface charge within these profiles. An increase in the soil solution ionic strength accompanying the fertiliser leaching front acts to increase the positive (and negative) charge density, thus providing adsorption sites for NO3. A decrease in the soil solution ionic strength occurs after the fertiliser pulse moves past a point in the profile, due to dilution with incoming rainwater. Nitrate is then released from the exchange back into the soil solution, thus buffering the decrease in the soil solution ionic strength. Since NO3 was adsorbed throughout the profile in this experiment it does not effectively explain the situation occurring in the field. Previous observations of the sulfate (SO4) profile distribution indicated that large SO4 accumulations in the upper profile may influence the NO3 distribution through competition for adsorption sites. A subsequent experiment investigating the effect of SO4 additions on NO3 leaching showed that NO3 adsorption was minimal in the upper profile. Adsorption of NO3 did occur, though only in the region of the profile where SO4 occupancy was low, i.e. in the lower profile. Therefore, the formation of the NO3 accumulations is dependent on the variable charge mineralogy, the variation of charge density with soil solution ionic strength, and the effects of SO4 competition for adsorption sites.