Potential availability of metals in anaerobic mono- and co-digestion of pig manure and maize

[Display omitted] •The metal speciation in digestates was influenced by those of the feedstocks.•The HMs bioavailability stabled around 24–37 days of mono- and co-digestion.•Zn, Mn, and Cd were predominantly bioavailable in mono and co-digestion.•Cr, Cu, Fe, and Pb were mainly linked with the oxidisable fraction. This study aims to analyse the potential availability of essential metals including as Co, Fe, Ni, Zn, Mn, and Cu and non-essential metals such as Pb, Cr, and Cd within anaerobic mono- and co-digestion of pig manure and maize. The metals partitioning was determined using the Modified BCR (European Community Bureau of Reference) sequential extraction at defined intervals over a 45-days period, correlating changes in metals speciation with key digestion variables. The findings revealed that Cr, Cu, Fe, and Pb were predominantly associated with the oxidisable fraction, while Zn, Mn, and Cd were potentially available in both processes. Notably, NH4+-N and the VFAs, except propionic acid, correlated significantly with the available fractions of Co, Mn, Ni, Zn, Cr, and Pb during mono-digestion of pig manure. The wider pH range and the chemical properties of the feedstock in co-digestion resulted in varied correlations between the metals availability and the digestion variables.