Long-term application of digestate and biofuel ash to arable soil : effects on the solubility of Cd, Al and trace metals as evidenced by simulated scenarios

Sammanfattning: The importance of nutrient recycling could not be emphasized enough in to-day’s society. Reduction of waste as well as recycling of nutrients would be the result if ash and biogas digestate could be used as fertilizer. However, the solubility of Cd, Al and micronutrients might be affected in a different way than by standard fertilizers. In this thesis, the effects of ash and biogas diges-tate on the solubility of Cd, Al, Ni, Zn and Cu were investigated on a long-term basis using the Visual MINTEQ model. As input to the model, geochem-ically active metals as determined by extraction were used. Batch experi-ments were made with the studied soils to provide data for model calibration. Values from literature were used not only for plant uptake, leaching and run-off, but also for the prediction of the long-term accumulation of soil organic matter (SOM) and exchangeable cations. Three scenarios were simulated; ap-plication of ash, digestate and a mix containing these two. The ash simulation showed an increase of the solubility for Cd, Ni, Zn, and Cu. Except for Cu this was caused mainly by an increase of exchangeable cations that led to stronger competition for sorption sites onto soil particles. The desorption of Cu was instead affected by the increase in pH following ash application. The digestate simulation showed a clear decrease in solubility for the same metals. This effect was caused by the addition of humic substances (HS) from the digestate, onto which the metals were sorbed. Aluminum behaved differently from Cd, Ni, Zn and Cu, the exception was the decreased Al solubility result-ing from the digestate application at Skedevi. The Al solubility was controlled by equilibrium with Al(OH)3, which explained the different behavior. The mix simulation did not substantially change the solid-solution distribution for any metal. The competition for sorption sites resulting from the ash applica-tion was balanced with the addition of HS from the digestate. The biggest source of uncertainty was believed to be the plant uptake and leaching of cat-ions. However, the results nevertheless give an indication on the change of the soil chemical environment due to ash, digestate and mix applications. As such, it shows that a mix could be preferable over single applications of ash or digestate if the soil conditions are to be kept unchanged.