Dispersed state evaluation by dynamic light scattering for alkaline suspension of black lioness fly frass (zoohumus) after its mineralization

Abstract

Alkaline suspensions of zoohumus from black soldier fly (Hermetia illucens) were studied before and after its additional mineralization. Four samples of zoohumus differing in processing stage and with pH from 7.6 to 11.7, were analyzed by Zetatrac (Microtrac Inc.) particle size and zeta-potential analyzer using dynamic light scattering (DLS) in the beat light spectroscopy (Doppler) mode. It was shown that all the studied samples were in a state of colloidal instability: the ζ-potential was +3.3…+3.8 mV at high ionic strength (I ≈ 55-65 mM, electric conductivity 5.4-6.2 mS/cm), which led to compression of the double electric layer to κ⁻¹ < 1.4 nm. Samples after mineralization (No. 1 and No. 2) were characterized by tri- and bimodal distribution of intensity with particle size from 100 to 6500 nm (and higher) at PDI of 2.418-2.960; the initial suspensions (No. 3 and No. 4) – by a bimodal distribution with particles size from 150 to 4000 nm at lower PDI (1.794-1.986). Samples 3 and 4 had higher concentration of organic carbon C_org (4.77-5.13 g/L compared to 3.17-3.43 g/L in samples 1 and 2) and exhibited pronounced organoleptic signs of active biochemical degradation. A diagnostic criterion for the completeness of suspension stabilization based on the C_inorg/C_org ratio is proposed: values ​​>0.32 correspond to a mineralized state, while <0.19 correspond to the original suspension. Additional mineralization is shown to reduce the risk of an unpleasant odor and increase the shelf life of the preparation. However, it is associated with a partial loss of humates due to their coagulative precipitation. The obtained results have practical significance for the development of technologies for the preparation of liquid organo-mineral fertilizers based on H. illucens zoohumus.