Diffusive gradients in thin films (DGT) as a robust and reliable technique to measure bioavailable metals in anaerobic digestates

Applying diffusive gradients in thin films (DGT) to digestates as a standardized method requires addressing unclear precision of the method and the lack of information on compounds in digestates responsible for metal binding. We tested the precision of the method on seven different digestates by measuring Co, Ni, Fe and Mn accumulated in the DGT devices. Next, we tested gels with two different pore sizes in parallel (restricted gel with smaller and open gel with bigger pore size) to identify organic complexes that bind trace metals. Finally, we tried to understand how the feedstock affects DGT concentrations. The precision of the method was acceptable as the RSDs of the DGT technical triplicates for Co were lower than 10% for all seven digestates. For the other metals, slightly higher RSDs were detected in a few samples: Mn RSD varied more than 10% in only one digestate sample (27% RSD), Ni RSD in two samples (19% twice) and Fe RSD in three (with 18% maximum RSD). The measurements of Co, Ni and Fe after diffusion through the restricted gel compared to the open gel showed a significant difference in maximum three out of seven samples. The investigated metals were found in higher concentrations on the DGT resins exposed to digestates originating from manure, energy crops and food waste than in DGT resins exposed to samples originating from wastewater treatment plant digesters. Overall, open DGT appears to be applicable for Co, Ni, Fe and Mn bioavailability measurements in digestates originating from various feedstocks.