Measurement of ion speciation in animal slurries using the Donnan Membrane Technique Original Research Article

The availability of nutrients in animal slurry for plant uptake depends on the total content as well as on the forms in which these nutrients are present in slurry manure. A DMT-manure cell was developed which can help to determine the speciation of nutrients in animal slurries. The cell consists of an acceptor compartment, which is separated from the slurry by two negatively charged cation-exchange membranes. The membranes only allow exchange of ‘free’ cations between the slurry and the acceptor solution. After about 4 days, Donnan equilibrium will be reached between ‘free’ cation concentrations in the acceptor solution and the ‘free’ cation concentrations in the animal slurry. The DMT-manure cell has been used to study the effect of dilution of animal slurry (with distilled water) on ‘free’ ionic species (K+, Mg2+, Na+, Ca2+ and NH4+). Total nutrient concentrations and the ‘free’ K+, Na+ and NH4+ concentrations decreased proportionately with increasing dilution. In contrast, the ‘free’ concentrations of Ca2+ and Mg2+ remained more or less constant upon dilution. The buffering of the ‘free’ Ca2+ and Mg2+ concentrations is most probably the result of Ca2+ and Mg2+ release from organic matter. Also, dissolution of phosphate minerals (struvite and whitlockite), which were likely present in the initial slurry, may have contributed to the buffering of the ‘free’ Ca2+ and Mg2+ concentrations.