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

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.