Bio fuel ash in a road construction: impact on soil solution chemistry.

Limited natural resources and landfill space, as well as increasing amounts of ash produced from incineration of bio fuel and municipal solid waste, have created a demand for useful applications of ash, of which road construction is one application. Along national road 90, situated about 20 km west of Sollefteå in the middle of Sweden, an experiment road was constructed with a 40 cm bio fuel ash layer. The environmental impact of the ash layer was evaluated from soil solutions obtained by centrifugation of soil samples taken on four occasions during 2001-2003. Soil samples were taken in the ash layer, below the ash layer at two depths in the road and in the ditch. In the soil solutions, pH, conductivity, dissolved organic carbon (DOC) and the total concentration of cations (metals) and anions were determined. Two years after the application of the ash layers in the test road, the concentrations in the ash layer of K, SO4, Zn, and Hg had increased significantly while the concentration of Se, Mo and Cd had decreased significantly. Below the ash layer in the road an initial increase of pH was observed and the concentrations of K, SO4, Se, Mo and Cd increased significantly, while the concentrations of Cu and Hg decreased significantly in the road and also in the ditch. Cd was the element showing a potential risk of contamination of the groundwater. The concentrations of Ca in the ash layer indicated an ongoing hardening, which is important for the leaching rate and the strength of the road construction.

Determination of low molecular weight organic acids in soil solution by HPLC.

An HPLC method employing an ion exclusion column was developed for the determination of low molecular weight organic acids in soil solution. The method includes extensive sample pretreatment using ultrafiltration and cation exchange. The method showed linear calibration graphs (r>0.99) and the limits of detection in the range 0.1-26 muM. The recovery of eleven added acids ranged from 89 to 102%. Soil solutions of five horizons of a podzolised soil were analysed. The results showed that these compounds made up 1-3% of the dissolved organic carbon and 0-14% of the acidity. Identification of the major acids was also carried out by capillary zone electrophoresis.

Aluminium speciation in forest soil solution--modelling the contribution of low molecular weight organic acids.

The concentrations of Al bound to identified low molecular weight organic acids (LMWOAs), phosphate and humic compounds in soil solutions (O1, O2, E, B1 and B2 horizons) from Fennoscandinavian coniferous forest soils were calculated. Two models were evaluated: WHAM (Winderemere Humic Aqueous Model) and a model treating the natural organic acids as monoprotic. The WHAM model was calibrated to find an average charge balance of unity, and then used for Al speciation. This was achieved considering approximately 80% of the natural fulvic acid to be 'active'. For the monoprotic model, constants obtained from previous fitting of soil solution data were used and the model was calibrated using acid/base titrations. The modelling confirms that the low molecular weight acids are important complex formers, binding on average 11-42% (O), 19-20% (E), and 0-30% (B) of the total Al in solution depending on the model used. The monoprotic model yielded higher concentrations than WHAM. Both models predicted that the major part of Al in solution was organically complexed to humic substances and LMWOAs, which was consistent with analytical values of 'quickly reacting' Al (monomenc inorganic forms; 12-14% of Al(tot) on average in the E and B1 solutions). Both models could predict the magnitude of the analytical values (RMSD = 4-5 microM). On average the WHAM model showed a better fit for the E horizon solutions and the monoprotic model for the upper B horizon. The pH dependence of the low molecular weight fraction of Al was modelled in the pH range 3-5.5 for a 'typical' O1 and E solution assuming no other changes in soil solution composition. The extrapolation showed that the models had a different pH dependence. The concentration of Al bound to LMWOAs in WHAM decreased with higher pH while the opposite was seen for the monoprotic model. Also the influence of the concentration of total Al in solution was modelled. The models showed similar trends but for the 'typical' O1 horizon sample the monoprotic model yielded higher concentrations of Al bound to LMWOAs than WHAM.