Zinc oxide nanoparticles (NPs) present in domestic wastewaters may accumulate in biosolids used as fertilizer. In this paper, metal complexation by typical biosolids is explored using methods from the humics literature. Uptake of Zn from NPs in the biosolids is evaluated. Finally, the kinetics of release of Zn species are reported as a function of (i) pH and (ii) the presence of strong binding ligands (e.g., ion exchange resin promoting release). The investigation revealed that (i) metal binding sites of biosolids are analogs of humic substances, (ii) ZnO NPs do not survive in the digestion environment, and (iii) any ZnO NPs dissolve to aqueous Zn in <10 d. Kinetics of Zn in biosolids revealed that Zn release is a function of biosolid protonation. At pH 8, Zn is retained in the biosolids, whereas at pH 4.5, 10% of Zn is released from the biosolids. Adding a chelating resin to the system at pH 5.0 led to Zn release from the biosolids as per Noyes-Whitney kinetics, releasing 85% of the bound Zn in 360 h. Fifteen percent of Zn appeared to be irreversibly bound.
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