Properties of the DOM in Soil Irrigated with Wastewater Effluent and Its Interaction with Copper Ions

DOM samples extracted from a wastewater effluent (EW) and from leachates collected from lysimeters in which Eucalyptus trees were deficit-irrigated with either EW (EL) or with water pumped from an aquifer recharged with the EW (TL) were studied. As ascertained by principal component analysis, DOM from both leachates displayed similar properties, different from those of the EW DOM. ¹H-NMR and FTIR spectra of the > 1-kDa DOM size fraction of the EW revealed less aromaticity than this fraction of either leachate. 3-D fluorescence maps indicated that the density of nitro, carboxyl, or phenol groups attached to aromatic structures was lowest in the EW DOM and only that DOM displayed protein-like fluorescence peaks. The leachates’ DOM > 1 kDa fraction complexed more Cu²⁺ per unit C than this fraction of the EW (Kd = ~ 10⁵ and ~ 10⁴ L kg⁻¹ DOC, respectively, at ~ 10⁻⁵ M free Cu²⁺). While Cu complexed preferentially with non-fluorescing sites in the EW DOM, in the leachates’ DOM, Cu bound primarily with fluorescing (aromatic) groups or with adjacent groups. The similar behavior displayed by DOM from leachates obtained under irrigation with either EW or reclaimed water suggests that processes occurring in the soil (e.g., by roots or microbiota) influenced the soil DOM’s properties to a larger extent than the nature of OM in the irrigation water. Thus, irrigation with good quality secondary effluent should not significantly enhance heavy metal mobility as compared to their mobility under irrigation with higher quality water.