Impacts of EDTA on the fate of nutrients and heavy metals during the hydrothermal carbonization of poultry manure

Hydrothermal carbonization (HTC) has demonstrated high potential for livestock manure management. However, reducing the potential risk of residual heavy metals is still challenging. In this study, poultry manure (PM) was hydrothermally carbonized to investigate the impacts of HTC temperature and ethylenediaminetetraacetic acid (EDTA) on the evolution of nutrients (i.e., N, P, K, Na, Ca, and Mg), heavy metals (Cu and Zn), and the properties of hydrochars and process water through HTC. Results indicated that HTC temperature was a critical factor that regulated hydrochar and process water properties and the fates of nutrients and heavy metals, while EDTA mainly affected metal distribution through HTC. A high proportion of C, N, and P was reclaimed in the solid phases, making hydrochar an optimal organic fertilizer with high fertility (N + K2O +P2O5 > 5%). However, N and P in hydrochars were low in availabilities. Through HTC, Ca, Mg, Cu, and Zn were concentrated in hydrochars, but Na and K were mainly distributed in the process water. A rising HTC temperature elevated the proportion of Zn in hydrochars, while it had limited impacts on Na, K, and Ca distribution. By comparison, EDTA facilitated the release of Cu, Mg, and Zn into the process water. Furthermore, EDTA involvement reduced the potential risk of Cu and Zn in hydrochars. Our findings highlighted the sound impacts of EDTA on risk reduction and nutrient recovery from livestock manure through HTC, and would provide references for livestock manure management for sustainable agriculture.