Cd, Cr, and Pb concentrations increase during composting: implications of uncertain mass balance and analytical recovery for compliance sampling.

Metal concentrations during composting reflect changes in dry mass, metal inputs or losses, and the fraction recovered by digestion. Concentrations of Zn, Cu, Fe, Mn, Ni, Cr, Cd, and Pb were measured over 90 days in five poultry litter (PL): vegetable waste blends and analyzed under three assumptions about dry matter loss. Cd in pure-PL compost increased from 0.84 to 3.34 mg kg-1 dry matter. After correction for the loss inferred from Zn and Mn, mean apparent recovery factors were 1.03 for Zn, Cu, Mn, and Ni and 5.11 for Cr, Cd, and Pb (95% bootstrap interval 3.05-8.23). The latter group remained above unity even at 50% assumed loss. Assigning the Fe increase to soil incorporation gave 0.13% mineral addition, accounting for about 4% of the Cr increase and less than 1% of the Pb increase under simplified mixing assumptions. First-order fits gave a mean rate constant of 0.0143 d-1, corresponding to a half-time of 49 days. Increasing digestion recovery could explain these patterns, but unmeasured dry matter loss, metal transfers, and analytical accuracy prevent confirmation of that mechanism or constant metal mass. Day-90 Cd exceeded the EU organic soil improver limit of 2 mg kg-1 dry matter in the two most PL-rich blends. The uncertainty model estimated a 5% exceedance probability at approximately 48% PL. These findings support sampling the finished product, assessing maturity independently, and validating the digestion method.