Trace element release from forest floor can be monitored by ion exchange resin tubes

Risk assessment of heavy metal input into forest ecosystems requires information about metal fluxes from the forest floor (organic layer) into the mineral soil. Common methods for the monitoring of element fluxes are generally time‐consuming and expensive. Additionally, the reliability of the results is in part contested especially for trace elements, showing very low concentrations which are sometimes even below analytical detection limit. We used ion exchange resin tubes installed below the forest floor to determine heavy metal and As fluxes at 25 forest monitoring sites in Germany. Chloride tracer experiments and the comparison of our data with throughfall and lysimeter data, determined within the Level II monitoring network, proved the accuracy of our method. Mean trace element fluxes based on the resin method were 50 g As ha^–1 yr^–1, 2 g Cd ha^–1 yr^–1, 168 g Cu ha^–1 yr^–1, 176 g Ni ha^–1 yr^–1, and 186 g Pb ha^–1 yr^–1.The results show that the organic layer may change into a source of heavy metals after emission has decreased.     

Ammonia and Nitrous Oxide Emissions after Landspreading of Slurry as Influenced by Application Technique and Dry Matter-Reduction. II. Short Term Nitrous Oxide Emissions

Strategies reducing NH₃ volatilisation from slurry include separation of slurry, special application techniques and additives. We studied the impact of manure separation and application technique on N₂O release after manure application. Untreated and separated cattle slurry (dry matter content of 7.1% and 4.4%, respectively) was applied to winter wheat using broadcast and banded application and injection. The N₂O emissions were measured at high frequency for 14 to 20 days after slurry treatment by the closed chamber method. Manured plots showed significantly higher N₂O emissions than the control plots but neither dry matter reduction of slurry nor application technique significantly influenced the N₂O emissions. The variability of N₂O emission was influenced by the application technique and increased in the order: banded application – injection – broadcast application. There was no correlation between NH₃ losses from applied slurry and N₂O emissions. Thus reducing ammonia volatilisation will not necessarily increase N₂O emissions.     

Influence of the Soil Solution Composition on Retention and Release of Sulfate in Acid Forest Soils

The potential for sulfate retention is an important soil feature for buffering of atmospheric acid deposition. We studied the effects of increasing additions of different neutral salts and acids on mobilization and retention of SO₄ ²- in acid forest soils. Soils containing up to 11 mmol SO₄ ²- kg^-1 were equilibrated with H₂O, NaCl, MgCl₂, and HCl. Release of SO₄ ²- was highest with H₂O and NaCl additions and lowest when HCl was used. Increasing the ionic strength of the added solutions caused decreasing SO₄ ²- concentrations in equilibrium solution. Decreasing pH in equilibrium solution was found to be the reason for the decrease in release. Even when the pH was < 4, the SO₄ ²- release decreased. We assume that this finding resulted from the fact that in the soils studied the SO₄ ²- sorption was controlled by the high contents of Fe oxides/hydroxides. Experiments with Na₂SO₄, MgSO₄, and H₂SO₄ demonstrated that the B horizons already containing high amounts of SO₄ ²- were still able to retain SO₄ ²-. Sulfate retention increased in the order Na₂SO₄ < MgSO₄ < H₂SO₄, which corresponds to increasing H⁺ availability. The higher SO₄ ²- retention along with MgSO₄ compared to Na₂SO₄ may be caused by higher potential of Mg to mobilize soil acidity compared to Na.     

Mechanism of molybdenum sorption to iron oxides using pressure‐jump relaxation

It is widely accepted that the fixation of oxyanions is due to diffusion of the ions into the pores and interdomains of iron oxides. Most studies have used batch techniques, which do not allow to clearly differentiate chemisorption from mass transport phenomena. Thus, it is not yet clear, whether strengthening of chemical Mo bonding occurs along with residence time, in addition to diffusion processes. In this study we used pressure jump relaxation (p‐jump), a very fast kinetic technique, to (1) elucidate the Mo/goethite interaction and to (2) analyze the effects of aging the Mo/goethite complex on Mo chemisorption. A synthetic goethite was incubated with Mo solution (1 mM Mo) for 12, 24, and 72 hours at pH 4. At the end of the incubations p‐jump experiments were performed on the suspensions at temperatures ranging from 283 to 303 K. Relaxation kinetics were modelled using a combination of two first order terms. In addition, the amount of Mo sorbed to the goethite after different incubation times was determined by graphite furnace atomic absorption spectroscopy. The MoO₄/goethite systems revealed a fast relaxation time (= reciprocal of rate constant, about 4 ms), that decreased with increasing temperature and a slow one (about 60 ms) that did not depend on temperature. Activation energy of the fast process was 76 kJ mol^—1. We did not observe any effects of incubation time on the fast process. However, the amount of Mo sorbed to the iron oxide increased with increasing incubation time. We conclude that the fast relaxation represents Mo chemisorption to the goethite. Slow relaxation seems to be due to Mo transport within the suspension. The pressure jump results indicate, that the dominant surface species of Mo sorbed to goethite do not change along with residence time.     

Water flow patterns and pesticide fluxes in an upland soil in northern Thailand

Rapid percolation of water through soil facilitates both the recharge and the contamination of groundwater reservoirs. We have studied the variation of water flux and pesticide leaching through a soil in northern Thailand. At a depth of 55 cm, two pits were equipped with tensiometer-controlled glass suction lysimeters that were connected to a novel on-line solid-phase extraction device. Nine insecticides varying in water solubility from 10⁻² to 10⁺⁶ mg l⁻¹ were applied on the soil surface, and leaching was monitored for 8 weeks. Measured water fluxes were compared with simulated values. Total recovery ranged from traces (malathion, triazophos) to 1.3% (dimethoate) of the applied amount, showing a decreasing retardation with increasing polarity of the substances. All pesticides were detectable in the soil solution during the first rain after application. Due to fingering, 83% of the leachate was transported through 38% of the area at leaching rates of < 2 mm per day. A new adaptation of the Simpson Index revealed that the diversity of the flow pattern increased exponentially with decreasing rates of seepage water flux (R² = 0.80). No such correlation was found when leaching was faster, indicating that the flow pattern switched from a fingering- to a matric-dominated flux. No long-term leaching of insecticides was observed. The two profiles studied behaved similarly in terms of both water and pesticide transport. Therefore we suggest that the flow pattern is a stable property of the soil that can be accurately described by our combination of novel experimental setup and statistical analysis of the flow field.     

Biochar addition enhanced growth of Dactylis glomerata L. and immobilized Zn and Cd but mobilized Cu and Pb on a former sewage field soil

The mobility and bioavailability of heavy metals in soils contaminated by irrigation with wastewater increase with increasing mineralization of accumulated organic substance and decreasing pH. In laboratory experiments addition of biochar reduced heavy metal availability to plants and enhanced plant growth. However, literature from field trials is scarce. Therefore, we conducted a 2‐year field experiment with orchard grass (Dactylis glomerata L.) to study the effects of miscanthus‐derived biochar applied to sewage field soil on biomass production and concentrations of zinc (Zn), copper (Cu), cadmium (Cd), lead (Pb) and various nutrients in plants and in the soil solution. Biochar was mixed into the contaminated topsoil (30 cm) with addition rates of 0, 1, 2.5 and 5% on a dry‐mass basis (g 100 g^?1). The soil solution was collected with suction plates installed at a depth of 30 cm. Addition of biochar increased biomass production and reduced Zn and Cd concentrations in the soil solution. Zinc concentrations were also reduced in plants. This effect seems to be attributable to an increase in pH caused by biochar addition. In contrast, Pb and Cu concentrations in the soil solution generally increased and were related to the concentrations of dissolved organic carbon (DOC). Copper concentrations also increased in the plants; however, only at the beginning of the measurement period. Our data indicate that increased concentrations of DOC, Cu and Pb in the soil solution might be a transient effect. Therefore, further research is needed to determine the long‐term effect of biochar amendment on element immobilization and leaching into groundwater.     

Novel ferrihydrite sand coating process as a first step for designed technosols

Purpose

The production of technosols to remediate polluted or sealed urban soils to sustain new green areas is mainly empirical. For this, our research aims to contribute with the scientific knowledge base for purpose designing of technosols. Since iron minerals play an important role for many different functions of soils, we simplified a technique to incorporate and stabilize iron minerals in a substrate: a sand coated with an amorphous iron (hydr)oxide, a 2-line ferrihydrite (2L-FH).

Materials and methods

The 2L-FH was precipitated by neutralization of a concentrated FeCl₃ solution. The suspension was homogeneously mixed with the sand and the mixture was dried at 35 °C. The mechanical stability of the 21 2L-FH-coated sand was determined by shaking the aggregates in water for 0, 1, 10, 100, and 1 000 min. The degree of coating detachment and the properties of the coating after shaking were characterized through (a) Fe content, (b) zeta-potential and particle size of the detached particles, (c) the specific surface area (SSA) of the coated sand, and (d) its surface structure using scanning electron microscopy (SEM). A phosphate adsorption isotherm was performed to measure the P-sorption capacity of the shaken samples and to test the 2L-FH-quartz attachment stability against the surface charge reduction of the 2L-FH associated with P adsorption.

Results and discussion

A reduced Fe loss (30 %) and smaller sizes of the coating detached particles in the sample shaken for 1 000 min indicate that a fractioning and reattachment of these aggregates occurred during the agitation process, resulting in a smoother surface (SEM), and a larger SSA and P-sorption capacity. The coated shaken samples showed P-adsorption capacities (5.3–6.34 μmol P g^?1) comparable to high loadings of phosphate in soils, and low detachment of Fe (7–14 %) in spite of negativity surface charge increase.

Conclusions

The practical novel coating process along with the 1 000-min shaking produced a mechanical resistant and P-adsorptive effective coated sand that could sustain the needs of plants in further experiments.

     

Coating a dredged sand with recycled ferrihydrites to create a functional material for plant substrate

Purpose

Urban greenery provides a series of benefits for the environment and inhabitants of cities. However, the substrate preparation mostly implies the mining and erosion of valuable natural soils (e.g., peat). Purpose-designed substrates, preferably made of waste materials, could avoid the extraction damage. The present work aims at improving the production and lowering the costs of a functional stably coated sand with ferrihydrite. This functional substrate combines the Fe (hydr)oxide sorptive capacities and the fast drainage of sand. Thus, secondary raw materials were tested: a dredged sand and three Fe (hydr)oxides; one from groundwater, an industrial intermediate product, and a mining by-product.

Materials and methods

Three Fe (hydr)-oxides were structurally characterized by XRD, XRF analysis, and SSA measurements. Further, amorphous Fe (hydr)oxide concentrations were determined. Sludges of these Fe (hydr)oxides in different concentrations were hand-mixed with a dredged and a mined sand, and dried at 35 °C. The stabilization of the coating was made by heavy shaking (250 rpm) the coated sand with water (3:1 w:w) for 0, 10, and 1000 min, washing and drying at 35 °C afterwards. Thereafter, the effectiveness of this treatment was determined by the Fe concentration and pH of the coated sand, along with the particle size of the detached aggregates during shaking, and the pH in the washing water. The morphology of the coating was observed by scanning electron microscopy.

Results and discussion

All Fe (hydr)oxides were 2-line ferrihydrites with large SSA, and coated both sands. Only after 1000 min shaking, homogeneous and small ferrihydrite aggregates covered the sands surfaces (verified by SEM and particle size). The impurities of the ferrihydrites affected the stabilization of the coating. Calcium carbonates enhanced the aggregation and reattachment of the Fe aggregates to the sand during shaking, while phosphate reduced the reattachment by stabilizing the aggregates in the suspension.

Conclusions

Two out of three ferrihydrites were suitable to develop a stable coating. To coat dredged sand with both ferrihydrites lowers the cost and production time to obtain a functional substrate. One ferrihydrite has a high pH due to its high CaCO3 content, and sand coated with it may be used as an amendment for acidic clayey soils.