Stefan Gäth

Sorption and bioavailability of heavy metals in long-term differently tilled soils amended with organic wastes

In this study, bioavailability and enrichment studies were focused on Cd, Zn and Cu in two different soils (sandy loam and silt loam), which were treated with compost and sewage sludge, respectively, both under conventional tillage (CT) and no tillage (NT). Long-term NT resulted in a significant increase in Cd and Zn extracted by aqua regia in a soil profile (0-25 cm), especially in the 0-3 cm layer. In the sandy loam, a slight increase in cadmium due to sewage sludge application was observed. Results from EDTA-extractable heavy metal analyses were not consistent but NH(4)NO(3)-extractable concentrations were significantly lower in the NT soil. Bioavailability of heavy metals was reduced in the long-term NT, resulting in lower uptake of Cd and Zn by plants and accumulation in the soil. The effect of tillage on enrichment of heavy metals in soils was more significant than the impact of the fertilisation by means of legally allowable application rates of organic wastes. This work suggests that non-tilled soils enriched in heavy metals should not be amended with organic waste in the long term.

Comparison of decision support systems for an optimised application of compost and sewage sludge on agricultural land based on heavy metal accumulation in soil.

Two different decision support systems (DSS) for the application of organic wastes on agricultural land were developed. Both DSS calculate the maximum application period of organic waste considering exhaustion of the uptake potential of soils for heavy metals. The definition of the uptake potential differs between the DSS alternatives. In the first DSS (DSS-AR), the uptake potential is derived from the difference of actual total heavy metal concentration in soil (according to aqua regia digestion) and the respective statutory limit value. The other DSS (DSS-SI) calculates the remaining sorption capacity of the soil for a heavy metal, i.e. the difference between the predefined maximum and the actual heavy metal concentration at the sorbent. The concentration of sorbed heavy metal is derived from pedotransfer functions (general purpose Freundlich isotherms) using predefined limit concentrations in soil solution (WHO drinking water quality standards) or the actual concentration of soluble heavy metal (according to neutral salt extraction), respectively. For evaluation of their individual characteristics, both DSS were tested in model scenarios using soil data (basic physicochemical properties; Cd, Pb, and Zn concentrations) from various agricultural regions and German guidelines for organic waste application. The DSS-SI showed a better performance than the DSS-AR in this context. The capacity of the soil for heavy metal uptake was used with higher efficiency, i.e. the potential was maximised while environmental limit values were still complied with. Furthermore, the DSS-SI offered a better approximation of the natural variability of soil conditions using an extended set of soil properties in comparison to the DSS-AR. Despite these indications of good DSS-SI performance, the approach requires improvement with regard to the pedotransfer functions implemented in order to consolidate and extend the range of predictions. Furthermore, it should be noted that assessment of the DSS-SI is influenced by the availability of data and is only valid under the current environmental and statutory framework at the regional scale observed. For this reason, transfer of the results of this study into regions with other administrative structures and/or environmental conditions is critical. Individual testing of the DSS-SI and the DSS-AR is required prior to the intended transfer, i.e. implementation of the approaches in the decision support process.

Regionalization of sorption capacities for arsenic and cadmium

To fulfill the purpose as a sink for trace elements, soils must not be overloaded with As and Cd. Therefore, it is necessary to get knowledge of the sorption capacities of soils on a regional scale. The determination of these sorption capacities for large areas is, however, impeded by the great expenditure of laboratory work involved. With data presented here retention capacities for cadmium and arsenic from routinely determined soil parameters are estimated. In batch experiments the sorption behaviour of 40 soils from the area of Freiberg/Saxony in Germany was examined. The obtained sorption isotherms from the laboratory were fitted to the Freundlich equation (S = k*Cm). The two constants (k, m) of this equation were used for multiple linear regression to correlate the sorption capacity and the soil parameters, namely clay content, pH value, total organic carbon, and dithionite extractable Fe contents. Due to long lasting ore mining of Freiberg there exist high background levels in that area for the two surveyed elements As and Cd. Therefore, this study offers two different mathematical procedures to take these contaminations into account. Thus the experimental data were corrected before they were fitted to Freundlich and pedotransfer equations were determined. Using the transfer equation, parameter k and m for cadmium sorption could be estimated with statistical certainties of 91% and 61% (adjusted R2), respectively, whereas the predictability for the arsenic sorption is not practicable because achieved R2 values are very low (17% and 7%).

Improved cadmium sorption isotherms by the determination of initial contents using the radioisotope 109Cd

Cadmium (Cd) sorption isotherms were estimated by two different analytical approaches to assess the influence of initial Cd concentrations of soil matrix on the sorption of added Cd. For the laboratory experiments a heterogeneous set of samples was collected to include a wide range of different initial Cd concentrations. Comparison of both analytical methods (conventional analysis, radioanalysis) resulted in a strong conformity of Cd contents in solution at equilibrium. The calculated Cd concentrations in the soil solid phase differ according to the analytical approach for considering the initial contents. The determination of the initial contents by the proposed radioanalytical method with 109Cd resulted in long linear Freundlich-isotherms, even in the low concentration range. Thus, radioanalysis seems to be the most suitable method to recognise the initial contents of Cd in soil. EDTA extractable Cd represent the initial concentrations, which are averaged over solid and liquid phase. However, depending on the sorption characteristics of the soil these rates vary. In the investigated set of soil samples 52.3 to 99.3% of Cd must be added to the solid phase. Erstellung verbesserter Cadmium-Sorptionsisothermen mittels 109Cd unter besonderer Berucksichtigung der Ausgangsgehalte  Unter Verwendung zweier verschiedener analytischer Methoden wurden Cadmium (Cd)-Sorptionsisothermen erstellt, um den Einfluss der Ausgangsgehalte von Boden auf die Sorptionsversuche zu untersuchen. Zu diesem Zweck wurde ein relativ heterogenes Probenkollektiv zusammengestellt, welches ein breites Feld verschiedener Ausgangsgehalte erfasst. Ein Vergleich der beiden Analysenmethoden (konventionelle Analytik, Radioanalytik) resultierte in einer sehr hohen Ubereinstimmung der Cd-Gehalte in der Gleichgewichtslosung. Die errechneten Cd-Gehalte in der Festphase zeigten jedoch Unterschiede entsprechend dem gewahlten Verfahren zur Berucksichtigung der Ausgangsgehalte. Tragt man die an Freundlich angepassten Sorptionsisothermen doppelt logarithmisch auf, so zeigen nur diejenigen einen log-linearen Verlauf, die mittels Radioanalytik erstellt wurden. Dies deutet darauf hin, dass das radioanalytische Verfahren zur Anrechnung der Ausgangsgehalte besser geeignet ist als die bislang angewendeten konventionellen Analysenmethoden. Danach muss die EDTA-Fraktion im Boden als Ausgangsgehalt bei der Erstellung der Sorptionsisothermen berucksichtigt werden, wobei je nach Sorptionscharakteristik des Bodens Anteile der EDTA-extrahierbaren Gehalte sowohl auf die Fest- als auch auf die Losungsphase aufgeschlagen werden mussen. Bei dem hier ausgewahlten Probenkollektiv liegen die auf die Festphase anzurechnenden Anteile an EDTA-extrahierbarem Cd zwischen 52,3 und 99,3%.

Tillage effects on the accumulation of polychlorinated biphenyls in biosolid‐amended soils

Soil tillage along with the application of organic waste probably affects the concentrations of organic carbon and the enrichment of introduced polychlorinated biphenyls (PCBs). In a three-year experiment the PCB status of soils from three different field sites (silty clay loam, silt loam, sandy loam) which were long-term differently tilled (NT = no-tillage, CT = conventional plough tillage) and amended with two different organic wastes such as sewage sludge and compost (biosolids) was examined. No significant alteration in soil-PCB quality and quantity with biosolid application could be proven within the course of the experiments. This indicates soil-air exchange of PCBs dominates their concentrations in soil. Organic carbon in soil was significantly tillage-dependent and determined the fate of PCBs resulting in a generally elevated PCB-level in the non tilled soils. Linear regression of PCB load and organic matter content of all investigated untreated soils was highly significant (R2 = 0.73). Due to already elevated PCB levels in non tilled soils with a maximum of 65 μg kg—1 in the superficial layer of the silt loam control plot, any additional potential input, i. e. through the amendment with organic wastes, should therefore be avoided. Anreicherung von polychlorierten Biphenylen in Boden unter dem Einfluss der Bodenbearbeitung und der Anwendung von Bioabfallen Die Anreicherung polychlorierter Biphenyle (PCBs) im Boden wird von dessen Eigenschaften, wie z. B. dem bearbeitungsabhangigen Gehalt an organischer Substanz oder der Zufuhr organischer Masse mit Abfallen bestimmt. In einem dreijahrigen Feldversuch wurde der PCB-Status langjahrig differenziert bearbeiteter (konventionelle Pflugbearbeitung gegenuber Direktsaatverfahren) und mit organischen Abfallen (Klarschlamm bzw. Kompost) gedungter Boden (schluffiger Ton, toniger Schluff, schluffiger Sand) gepruft. Die Anwendung der Abfalle fuhrte innerhalb des Versuchszeitraums zu keiner signifikanten Erhohung der PCB-Gehalte oder Veranderung der PCB-Muster in den Boden. Deshalb kann die Annahme, PCB-Gehalte im Boden wurden uber den Austausch Boden-Luft masgeblich mitbestimmt, bestatigt werden. Der Gehalt an organischem Kohlenstoff war signifikant vom Bodenbearbeitungssystem abhangig und fuhrte zu einer Anreicherung der PCBs in unbearbeiteten Boden. Die lineare Regression der PCB-Belastung mit dem Gehalt an organischem Kohlenstoff der untersuchten unbehandelten Boden war hoch signifikant (R2 = 0,73). Aufgrund bereits erhohter PCB-Gehalte in langjahrig unbearbeiteten Boden — mit einem Maximalwert von 65 μg (kg TS)—1 in der oberflachennahen Schicht der Kontrollparzellen des tonigen Schluffs — sollten dort weitere Eintrage mit organischen Siedlungsabfallen vermieden werden.

Sorption of Cd in Soils: Pedotransfer Functions for the Parameters of the Freundlich Sorption Isotherm

Using a data set derived from 480 soil samples, pedotransfer functions for the coefficient (K) and the exponent (m) of the Freundlich sorption isotherm were developed. The functions were validated with data from 135 independent soil samples. We found high goodness-of-fit for the approaches of the Freundlich coefficient, whereas the efficiency of the models for predicting the independent samples should be improved. For the Freundlich exponent, the mean value of the parameterisation data set (m = 0.88) served as a better predictor than any of the developed functions. These findings contribute to the interpretation stated by previous authors that variation of the Freundlich exponent is of minor importance and therefore negligible in the course of sorption modelling.

On the Use of Different Efficiency Criteria for the Validation of a Heavy Metal Balancing Tool

The validation of metal balancing tools is usually based on the comparison of simulated versus observed data. In our study, we applied a set of different relative and absolute criteria to evaluate the performance of the model Assessment Tool for Metals in Soils. In this process, the uncertainty of the model output and the sensitivity of model parameters were also assessed. The study includes data from 123 agricultural used top soils which are characterized by the application of different fertilizers (mineral and farmyard fertilizers, sewage sludge) resulting in diverse metal inputs into the soil. Although the most common validation criteria (coefficient of determination, error ratio between prediction and observation) indicated a good model performance in predicting the metal contents over a simulation period, the absolute measure (mean absolute difference between prediction and observation) showed that the informational value of the validation results was limited for several sites. Therefore sites with short simulation periods and/or low metal inputs are not suitable for validation, because the model uncertainty covers the metal concentration changes. Excluding such sites from the validation statistics led to evaluable and quite better validation results. Although the calculated output uncertainty was low, a further reduction can be realized by improving the database for the identified sensitive parameters (initial soil metal content and fertilizers metal concentration).

Sorption Behavior of Uranium in Agricultural Soils

Because mineral phosphorous fertilizers are often loaded with uranium (U), their application could cause a significant U contamination of agricultural soils. In order to predict the mobility and plant availability of U, it is necessary to describe the liquid/solid distribution of U in soils. First results indicate a high adsorption of U onto the soil matrix. Freundlich adsorption coefficients (log Kf) range between 1.3–6.7 and illustrate the different sorption capacity of the investigated soils.

Uranium in Secondary Phosphate Fertilizers and Base Substrates from Water Treatment Plants

As a limiting factor for plant growth the demand for phosphorous (P) fertilizers is huge while the global phosphate reserves are strongly limited. Thus, the recycling of P from secondary raw materials such as sewage sludges and ashes or other organic waste materials will be a more and more important resource. In fertilizers, both primary and secondary, not only the nutrients but also the content of pollutants is important for an application in agriculture. Mineral P fertilizers may contain remarkable concentrations of uranium (U), cadmium (Cd) and other trace elements depending on their origin and chemical treatment.